Yes, but not as therapists. Sometimes it takes a friend to get a depressed person to a doctor. Depression that was once thought to be primarily psychological in nature is now thought to be largely physiological. Treatment is more likely to be with medication than counseling, although sometimes both are necessary. Let the doctor decide.
Friends can provide a support system during a time of physical treatment.
Many forms of depression are best treated by achieving of chemical balance. This may take some time, during which support and care can be essential. Where the depression is primarily emotional, friends can do a lot of listening. Friends should not be used as therapists. However, when someone who knows you well suggests that you need more help, you probably do.