This. Yes a depressive disorder does make a person selfish in that they are absorbed in negative thoughts, are pessimistic, and often lose interest in personal relationships and other things that once gave them pleasure.
It is also true that treatment, especially cognitive/behavioral therapy, is most effective when the patient is invested and committed. As a friend, however, you need to be involved and let that person know that you are always there for them. And if they are expressing suicidal ideation, or are engaging in acts of self-harm, you need to contact people who have the power/influence to directly intervene.