There is a lot of advice, and I can only give my version.
I've had a friend who was accused of stealing videogames from us. We all were suspicious about him, and just like you, we did not ask him about it. We simply had no hard evidence, and we were afraid that if we are wrong, we lost him as a friend. Just like you, I trusted all the other friends, and I wasn't so sure about this guy.
The only solution was to catch him red handed, but we didn't want to risk losing another videogame. Also around the 2000's, we did not have easy and cheap camera solutions like we have now. Eventually, the friendship ended, because we lost interest and trust in him. A few months later another friend - let's call him friend no. 2 - bailed on us, out of the blue.
A few years ago I received a picture from my best friend, showing friend 2 having a garage sale. Surprisingly he was selling literally every videogame we lost.
Turns out it wasn't friend 1, but friend 2 who stole our games. When we evaluated this, it turned out that the rumour of friend 1 stealing, was spread by friend 2. He turned out to be the thief. So, no matter how much you trust someone, things can take a weird turn. We did not take any action, because it was too long ago and we did not want to open a can of arguments. We lost a friend because of early judgement.
I don't understand why a collective amount of $600 !!! was lent to your friend though, because at a certain point you had to stop lending money in order to have a chance to get it back. In this case, there's a little blame in every one of you. There is nothing wrong to have a stern chat with him about this, though.
But if you want to lose your friends (and eventually your lent money) then it would not be wise to confront him being a thief. He will just leave you guys.
Just make sure, when he is around, that you will not leave him alone. If you can't, a camera might help because when you really want to confront him with this, make sure you have proof. (Also make a copy of the footage in case he steals the recording.) This proof also helps you for him not bailing on you, since you can make it clear that you are filing a report with the local police department, if he tries to run away. And to be honest, if you have evidence of your friend stealing, is he still your friend?