He said his friend had paid someone to meet him at the place where he first met Yu. He didn't see the person his friend paid for. Only Yu was standing there so he jumped to the conclusion that Yu is a sex worker.
You are probably right, but he never said anything about any payment. But the sentence "Don't worry, he's definitely clean" would at last make sense if he was talking about a sex worker.
Its in the 26 chapter... "Introduced" or "arranged" are common slangs for paid or hired in countries where prostitution is illegal. In my country, it's "arranged for ~friendship~".
You’re really not thinking straight when you have a crush, how the fuck did he come up with the conclusion Yu was a sex worker