Mormons are very sensitive to imposing their beliefs/views on others. (Some who live in Utah might debate that ;-) - but that's a different discussion.) 
 
One of our "Articles of Faith" says "We claim the privilege of worshipping almighty God according to the dictates of our own conscience and allow all men the same privilege. Let them worship how, where or what they may." So we as mormons as a rule are very tolerant. 
 
I go out to lunch or dinner with lots of co-workers - none of whom are Mormon. When we go out - esp for dinner like on business trips - everyone else usually drinks. Even though I don't I don't feel uncomfortable in that situation. Now if I was in a situation where there is lots of drinking and people are getting drunk/loud/boisterous/etc - I would start feeling uncomfortable. I think most mormons are like that. I do know that some mormons get very uncomfortable being at dinner or around people where there is drinking, but I think that's more the minority. Everyone is different.  
 
You could just ask your friend in a side conversation. Given that you are frequently with him I suspect that he is ok. If he was really uncomfortable he would probably excuse himself from being in those situations. 
 
Hope that helps.