The only comfort I could offer is that they haven't lost their entire family. Your MIL still has you, she has YHWH and she has Jesus.
Personally I would tell her to be bold about her decision and not let anyone try to put her down. Give a bold witness about what she learned and state up fron that if they are going to let the doctrines of men come between family then something is really wrong with them.
The elders use the shunning as a weapon to get people who may have good reasons to leave, to stay, not out of love, but out of fear. Standing up to the elders and putting them in their place works wonders for inspiring others to do the same. Sure it will be hard at first, but as I said earlier, if enough people band together and oppose the Disfellowshipping procedure then the GB will eventually capitulate. The history of any nation or people clearly show that when enough people put their foot down, amazing things can be accomplished. Obviously she shouldn't cut her JW family out of her life, but go on encouraging them and loving them despite the doctrinal disagreements. Like the Bible says "So that they might be won over without a word."