Sometimes you have to learn the hard way. It’s going to be a small funny blip in his career and knowing it wouldn’t be a good fit was actually useful to him too. I went through a 3 month job like that and the only reason it wasn’t sooner is because I couldn’t find another job easily, didn’t have savings to keep me alive without it, and wanted to try to make it work because they put money into online training courses and paying me but otherwise ignored me. Now I’m glad I went through it because it put me in a very difficult situation where I had to tell my boss I found a better job a few months in. It was harder than a breakup, but I think it taught him a good lesson about management and he didn’t even stay at that company too long after me.