Of course you should tell him!! Holy shit you're friend's ex is working in a massage parlour!! And he wants to get back together with her. That is a huge deal and you need to tell him. He's your friend, not her. Even if they are apart and never getting back together, this is not a small development. You don't say if there are kids involved, or if he's paying alimony, or has some kind of legal restrictions on him due to the divorce. You may not know everything about it either. But something like this might mean a lot to the guy. Certainly, if he still pines for her or wants to reconcile, he should know what she's been up to. She may well lie to him about it - and yes you get to intervene. He's your friend, not some random guy at work.
There is a HUGE difference between a woman working in the sex trade and a guy (or a woman) having a one-night stand, picking someone up in a bar, dating etc. They've been apart for a year and its fine for them to be having sex with others. That's normal. But, sex in a professional capacity? Much different. Even different than him seeing an SP once or twice. She is WORKING in the sex trade. You know full well how society views that and you know full well this is a big deal. Disease, drugs, seedy pimps and MP owners, etc, not to mention the stigma and embarrassment. Its there, and its scary. And no its not the same to dip into that world for a couple hours a month, as it is to work in that world on a regular basis. The risks are greater.
The wonderful ladies in here are telling you to keep quiet about it. They either want to feel that their chosen profession is accepted by society and not a big deal, or they are seeing their biggest nightmare come to fruition (being outed, having a relationship ruined by their job). I wish it were normal and accepted, I really do. But its not, which is why we all use fake names and hide our faces, me included. Is your loyalty to a broader campaign to normalize the sex trade, or to your FRIEND? I would offer that informing your friend is more important. My perspective anyhow. Glad I'm not in your shoes.