Well, since I think no matter what the issues were in a divorce, apart from assault, battery, attempted murder etc., people who share kids of any age need to be able to be at least respectful of each other and this could possibly be an opportunity for that to happen.
Gut level, however, if your mom and grandmother were close, this is a time of grief for her that is probably not shared with him and adding a lot of additional pain to that is not appropriate. In your spot I would be honest with both. I would tell your mom that he will be with your brother and wants to pay his respects and is that acceptable to her? Hopefully enough time has passed that she will feel like she can handle it. If not, I would tell your dad how your mom feels about it and hope he will decide to do the more humane thing.
In the end they are both adults and they will have to make their own decisions, ugly scene or not. Your concerns of course are valid, but other than making sure your parents know what the deal is, you have no power in this situation.