We aren’t helping the situation. He’s already extremely pissed off and wants to punch this guys face off and honestly that won’t get him anywhere but thrown in jail with heavy fines and probably back on the surgical table for his shoulder. Calm heads prevail.
Catch more bees with honey. Take a few days to get your blood pressure back down and see what the issue was before figuring out course of action. We all make mistakes and sounds like this dude is spread thin and rushing jobs just to get on to the next one. That’s no excuse but being calm and collective to figure out the diagnosis is what needs to happen now. Then find out what needs to be done to rectify the situation is second. Explaining that you’ve been friends for 8 years and you value his skills and trust him to do what’s right is probably gonna get this solved faster than screaming and dropping F bombs. He can easily say it was from the machine shop or the used engine and not take any responsibility and bow out of it altogether. Then it goes to the next shop where it costs more money out of pocket and if that shop says it was from the first mechanic it turns into a small claims case and all this turns into years of aggravation and heart ache.
Your doing the right thing brother. Stay proactive but not over reacting. By giving this douchbag a few days to think about what he did wrong might work in your favor and if he has any morals or dignity he will make this right and do it in a timely fashion. You have enough on your plate right now to be getting worked up over something you can’t control. You trusted this guy and were friends with him for a reason. Let’s see if he values that friendship as much as you did. Hope this has a good ending, I’m pulling for ya bud.