If a loss is caused by a leaver, only the leaver should get a loss
If someone leaves everyone else should get loss prevented. This is so that trolls or acts of god do not have a negative impact on innocent players. This kind of randomness causes frustration and a feeling of helplessness, both of which lead to toxicity.
Some people say that this would cause people to game the system by leaving to help their friend. However, first of all people already game the system. Second of all, anyone repeatedly doing this should be banned and that solves that problem. Finally, this feature already exists in dota and while they may have this problem I don't think it's as big a deal as it is made out to be, and I think the good caused by this system outweighs the bad.
"That it is better 100 guilty Persons should escape than that one innocent Person should suffer, is a Maxim that has been long and generally approved." - Benjamin Franklin