That is a good point . he did prostitute her out but only a BJ. He did not want Nathan to fuck her if i remember correctly, he stopped her from doing that. So there are limits to his sharing i think. Besides you did not have to let Nathan get a BJ by Lily - you could stop the scene before she gets there => there are multiple endings.
Here are some paths/ending for Lily just on the top of my head:
1. could be Lily ends up with Fred married and all...
2. complete prostitute with Fred as the pimp
3. intelligent Lily who ends up running Castor or something like that (climbing all the way to the top using sex = the career whore)
4. she ends up with Mike - true love
5. loving wife - she stays with Nick
6... more ideas...