@Juan623 I don't know how co-host works, really, I've never needed one. But I've helped friends and neighbors out with guests when they were out of town, and property manage for a couple of friends. None of this has ever been a formal co-hosting deal through Airbnb or any other platform. I just charge them for my time, and they pay me directly. If this is a close friend, who you totally trust, as far as paying you well and on time, I don't see any reason why you couldn't just make some private arrangement. Altho many a friendship has dissolved over money issues.
I think it's a valid concern that someone who's already a host with good ratings and reviews would be apprehensive to have someone else's place affect their ratings.