I'm not that familiar with copyright law. Is he legally allowed to make a guitar cover of any song he wants to and distribute it? It's also not clear if there were ads on the videos. Would that play a role?
Thousands (millions?) of people post covers on YouTube with no issue, but sometimes people get unlucky. I don't think monetization makes a difference in the eyes of the law (maybe it would for proving damages? I'm not a lawyer.)
What's unusual is that he said he got 5 copyright strikes without telling him why. I've gotten a copyright notice on a video before and I got an email and I think it even shows on the video where the violation is. (I'm not 100% sure, it was a long time ago.)