This. You can stop being friends with someone for any reason at all. There is no obligation to remain in a relationship with someone if you don't enjoy their company, unless they're your minor child or something.

I think there also comes a point where there's a moral obligation to end the relationship. Regulus knew that, Sirius knew it, Lily (finally) knew it -- Snape didn't. And then Snape went on to gleefully join a murderous terrorist organization and collude in the attempted murder of a toddler.