I agree - if you're asked to be in the wedding, talk logistics then. If you aren't, wait for the invitation and see what it says.
And if sometime between now and the wedding you all talk about the wedding and there is a natural way to bring it up, then do so.
But I wouldn't make a point to pre-emptively go to your friend and tell her what happened w/ her mom. Just sit on it for now and see how it plays out.