I would hit the roof if I saw anyone throwing ANY dog and don't blame you for going off on her (I would too) but it sounds like you have a problem....this isn't what you want to hear - but you can't be there 24/7 so it just may be that he needs to be in a home without other males...
Neither of the dogs are at fault and while your roomie is trying to protect her dog ....calling him (Rocco) a piece of %#$ is immature but.... actually throwing him i
s abuse and a sign of what's to come
he could really get hurt even if he IS the aggressor.....She was wrong for throwing him. He could have been killed .. I'm glad he's ok
now you have to make sure he's never alone with her....
I take it you don't really know his history ? Were you aware he has issues with other dogs when you got him ? IF not - then I'd be trying to either find a new foster for him
or if your roomie moves out - maybe the issue will be resolved ...but I doubt neutering with alter this kind of behavior....it may tone it down but then again - maybe not.
Either way - all the best and I hope you can find a solution before this escalates even further.....he sounds like a little sweetheart but the agression problem has to be dealt with right away for his own good and for the sake of the other dogs in the home....