I would had to say that if your girlfriend was to leave the Church it would cause some tension within the family.  
 
In our faith families are eternally bound through the ordinances of the Temple, to leave the church is to leave these ordinances that bind families together. So her parents would very likely be heartbroken to see her leave.  
 
Now would they stop talking with her or ostracize her they should not though seeing that we are all human there pain at that loss could cause them to be more distant which is natural, but not right.  
 
The same thing happened because I joined the Church, my father disapproves and because of the strain that put on our relationship we talk much less not than we did in the past.

Just a short answer: No, Mormons do not believe in shunning. I have a couple of relatives who have left the Church. The rest of us are disappointed they have done that, but they are still family and we treat them just like the rest of the family. We still get together for holidays, birthdays, etc. If we did shun - that wouldn't be very "christian", would it.

Is ostracizing Christ-like no, but also realized that Christ was perfect and man is not.  
 
Ending ties though with friends and family that are wicked or apostates is not unChristians though. The Apostle Paul who spoke on behalf of Christ in the Ancient Church repeatedly spoke of not associating with the wicked. We are to love them, we are to reach out to them in service and show them the gospel of Christ but we are not to associate with them on a personal levee. Least we become tempted to stray from the straight and narrow path.  
 
In many cases what most call shunning is nothing more than the natural process of the mighty change that happens in one's heart when they are converted to the Gospel of Jesus Christ.  
 
I am still accused to this day of shunning friends and family because I will not attend parties with heavy drinking and music that does not invite the Spirit but instead is provocative and perverse in it's lyrics. It is evil to do this or against Christian standards absolutely not. To go to such events again is to invite sin into my heart again and Christ commands us to forsake that and to absolve such friendships if needed, through the Apostle Paul.  
 
Of course Jesus Christ himself would attend that party in a heart beat since he cannot be tempted with such things. So which Christ being perfect could do such things as to continue to associate with the wicked and there unrighteous behaviors, as men we must distance ourselves from those who desire to not consider our conversion to the Gospel and rather end a friendship than care enough to accommodate changes that one has made in there life. 
 
The same principal applies to those who leave the church and apostatize from the Gospel of Christ. I have been on that end as well. A few of the young men from my first ward have left the Church and no longer follow the standards of the Church. Living lives that I cannot associate with not because I no longer like them or have any concern for them but because the behaviors and standards they choose to follow are not compatible with the standards and behaviors of the Church so our interactions are very limited on that basis, see that while they were a member we had many things in common and now we have few things in common.  
 
So while it is correct that members of the Church will not prevent members who leave the Church for attending family events ect, ect. One must expect that the relationship will change while one is no longer a member of the Church if there lifestyle and behavior are no in accordance with the Gospel of Jesus Christ.  
 
This goes unto and including what many would call shunning or dramatically less interactions with friends and family that are still members of the Church, should the persons behaviors and lifestyle comes in deep conflict with the standards that members of the Church hold.  
 
While many will accuse this as shunning, it is not. One cannot continue to closely associate with a person or persons who will frustrate there eternal progression. To do so it to not be fully converted to the Gospel of Jesus Christ. Now when those who are not of our faith are accepting and understanding of the covenants and standards we follow one still can have very close and fruitful friendships and family ties. This is how it is with my in laws they are all catholic yet respect and cherish my faith in the Restored Gospel and we have a very positive relationship seeing that the standards of the LDS church and the Catholic Church are not different except on points of doctrine.  
 
Though those who wish to follow the ways of the world and live a life of excess and sin, it is almost impossible to maintain a active friendship with them, since they will not accept or respect your standards and you cannot your standards to accommodate there passions and desires least you risk your standing in the Church and your eternal progression.  
 
In the end if a member leaves the Church while there friends and family will love them no less and want nothing more than to see them be happy and again united with the Body of Christ, they will respect there choices and love them no less.  
 
Loving them no less though does not mean that they can accept any changes they make to there standards or lifestyles. Should those changes be in conflict with the standards of the Church and the covenants the members have made, then yes you can expect that interactions and associations with those friends and family will decrease maybe in some rare cases even stop. As we are taught throughout our lives in the Church was have the right to choose but not to choose the consequences of our actions. Should actions of a member that leaves the Church become in conflict with Gospel Standards that person will not be in close relationships with friends and family unless they make corrections.  
 
As I stated the same goes for those who convert to our faith. My actions (joining the Church of Jesus Christ of Latter-day Saints) have the reaction of my father not accepting my faith nor my choice to follow it. As much as he loves me dearly and I him our relationship suffers because our standards and beliefs are in constant conflict. While we do still talk on occasion we no longer have the deep relationship we once had. It is a painful sacrifice but one that I willing make because I follow the Standards of the Restored Gospel and my father follows the Standards of his Church.