You can't control how other people will respond. Even without the depression she would likely take the break-up badly because no parent wants to see their child in pain. Plus she was probably relying on you to care for him once they are no longer able. Don't tell her about the break-up until after you have done it, that wouldn't be fair to your bf to find out from his mother that she knew. And seriously think about blocking all communication after the break-up so that they can't manipulate you back into a relationship. Plus it would give them false hope that you might change your mind. Good luck