I think you definitely did the right thing in telling her OP. Sucks that it has ended this way, but I think you would find deep down she probably knows what you said is true. It makes me sad that women are stuck in relationships with dead beats like this. The fact that HE cheated on her and then had the audacity to place restrictions on her by not befriending her on facebook screams manipulative creep to me.
I think I would take a screen shot of the messages too like PP's said (if you still have them) and maybe just let her know you are there if she needs a friend.