A professional I know in the field (who repudiates drugs for CBT, and so is probably biassed, but still...) says that the lack of clarity over how an antagonist and a suppressant can both be claimed to operate on the same underlying problem and have adherents, points to bad understanding of what actually causes the problem.
Few drugs solely act to agonize or antagonize a single type of receptor in a single part of the brain. Many of these compounds do a lot of different things, some big and some small. Anti-depressants have very complex mechanisms of action which we don't fully understand, and may not understand for a long time. They are a rough and imprecise solution to what for some is an otherwise intractable and potentially fatal problem.
Obviously therapy and other techniques should be attempted before drug prescription, but your friend is grossly oversimplifying how these drugs work and how psychiatrists portray how they work.