Feynman himself casually notes Fuchs as "the fellow who turned out the be the spy", and despite their friendship never offered an skepticism about the conviction.

Fuchs confessed before he was prosecuted.

I'm not familiar with this particular case, but innocent people confess more frequently than you might think. Investigators put tremendous pressure on suspects during interrogation, and they can often feed details of the case to the suspect, resulting in a 'detailed confession'. Many times, the investigator is not intentionally trying to get a false confession.

That's not the case here and you can simply google the guy, his case is famous. He confessed voluntarily, likely because he was tipped off and knew he was already exposed. After serving his time he continued his life in East Germany. There is no doubt whatsoever he was a spy and he never disputed it when no longer under duress.

Good catch, I almost looked it up but I got lazy. I don't know if the Feynman bio I'm reading specified exactly who caught him but it didn't seem like there was ever any particular question whether or not he had done it but I can't either say that I know he was, I've just read it and not looked into the details myself.