Just loaded a game after getting the roommate, the sim show up in the picture, so dropping a roommate must be part of the regular move out system. Drat, no the load is broken, she is now a normal house hold member.
The rules for asking must be level of friendship and higher levels of "current interaction" (Your sim being social to the other sim, when chatting). Always had trouble with the friendly category, since a lot of the chat options drops the interaction level down, where romance and fun everything raise the level, unless you go outside the category.
Maybe free will get throttled on my machine, so it doesn't work well, because even with normal NPC sims, they are broken, there needs are all drained when you meet them in town or when they invite you to parties. (Parties has gotten better, used to be they where never home when you tried to click attend, that's fixed now, but I still get there and then get told the party is over, before I get anything done, because they are all working on eating/toilet visits)
Another problem I found: The Sim I invited to be a roommate, has a roommate listed in her friends list. :/
It look like roommate is broken without University. :/