Contrary to popular perception, men who use escorts are just like men who don't. Some of them are lovely people who very much respect women, are polite and helpful, well mannered and considerate. Some of them are you know, okay, you might not want to date them but there's nothing wrong with them. And a small proportion of them hate women and seek to abuse sex workers. So, exactly the same as every other subset of male population.
People get hung up on this thing of "he's bought a woman". No he hasn't. He's bought an hour (or whatever) of her services - just like any other service industry. When I pay my cleaner for an hour, have I bought my cleaner? No, I bought her labour. It's only our social conditioning that seeks to control women's sexuality that throws this stigma up, until we end up with "You can sell literally everything else you can do, you can sell your intellectual abilities, you can sell your physical labour, you can sell your soul so that you sit and take shit of awful customers and have to keep smiling. Oh and here's minimum wage. But don't you sell your vagina!! That belongs to your husband, or husband to be."
Incidentally, escort, prostitute, working girl, sex worker, whore are the exact same thing. "Prostitution" is a legal term more than anything else. I prefer "sex worker" because it reflects that I'm doing something that's actually work and deserves compensation.
The questions I'd be asking your BF would be around how did he choose the sex worker he saw - try to get a feel for if he had any concerns about her being trafficked. If he picked up a girl off the street then that would indicate he didn't have much ethics about making sure she wasn't being coerced. If he looked for an independent worker through advertising services then that may have (or may not have) been a factor for him. Most clients of mine express that they are uncomfortable seeing girls in brothels because they feel there is a higher likelihood of trafficked girls (which is correct.)
It's important to point out, again, that coercion and/or trafficking are significant problems within sex work, but they are a minority. The majority of sex workers are in charge of their own work and have chosen sex work over other careers for their own reasons.