I think that I would use our traditional monogramming guidelines. If he is referred to as Father______ then use the capital of that name. If he has multiple names; Father_____ _____ ______ then I would simply ask your friend to casually ask him with the multiple names how would they handle monograms. Best Wishes creating a sure to be treasured gift.
I knew I had seen this same question before. Have a look at the link. There are some answers there.
Maybe the person that wants you to do the monogram for him can tell you what monogram she wants!! That's a tough call!