21, parents divorcing, mother plans on moving in with me?

I think your friend's advice was probably best. Even though your mother is emotionally fragile right now, surely she'll understand how important your independence is for you right now, as someone moving out on their own for the first time. Just be honest with her and let her know how you really feel. It's so much better than shouldering the emotional and possibly material burden of having your mother live with you. Let her know that you love her and that you'll be there for her... just like your friend said, but stay firm and get out into the world on your own. Good luck!