I asked one of my firearms instructors (Matt Seibert) about this and his response was that the slide release is actually a slide stop and to chamber a round he always teaches to reach over the top and rack. He said one of the most common things he corrects on new students is to get them out of the habit of using the slide release.
He gave some good reasons, but I think in the end its up to you and how you are trained. As for me I never use the slide release while shooting, I did up until I started getting some quality training at Gunsite and other places now I always rack by reaching over the top.
The main reasons he gave for not using the slide stop as a slide release to chamber a round seemed pretty logical to me. He said that by pulling the slide back by hand you add a bit more tension to the spring and gain a bit more force to the release of the slide. He also pointed out that when you watch professional shooting competitions most shooters rack this way and very few use the slide release. Another reason he gave was that in a high stress situation its easier to grab the entire slide and rack it than risk fumbling for the tiny slide release and racking the slide is a more reliable way to chamber. The final reason he gave was that it was better for the gun, especially when closing the slide on an empty chamber you should always have your other hand on the top and gently let it close, never let it snap.
In years past when I used to use the slide stop as a release there were many occasions when the round did not fully chamber, especially as the gun got dirty. Ever since I started racking the slide I've never had the slide fail to close fully. So I'm convinced, but the way I see it if you want to use the slide release to close the slide, then use it. Its your gun, you're the one shooting it, do as you see fit. If you have an instructor that tells you not to, you can either do what he or she says or argue with them, I choose to just do what they say because I pay them to teach me how to shoot properly
Tell your friend "I'll do it my way, you do it yours" that usually works for me when I get into discussions like this with my shooting buddies.
Personally I'm on your side on this issue.
Good Luck!