You're in denial. You automatically equate sex work with a love affair or 'having a relationship with someone else - that is, not monogamous'. Sounds like you even equate sex with love or infidelity.
I don't understand your point about me calling the OP a hypocrite. When I said that his friend is a hyprocrite, it's because he can enjoy hobbying with impunity while remaining secret, discreet and possibly dishonest about it, yet when it comes to the opposite sex who is providing the service, she's not entitled to secrecy or discretion. Hence the hypocrisy. However, I get that it's not about the OP, it's about HIS friend who supposedly doesn't hobby that he thinks he's protecting.
All that being said, I think what maybe you're getting at is not her 'cheating' per se, but her being dishonest about her sex work. In a relationship, we hope for trust and honesty, and no secrets (OK, there may be secrets from the past that are better left unsaid). If you feel she should be honest with him, then that's a better reason to argue for disclosure.
However, as Allison as pointed out, we don't know what's going on in their relationship. This is what my own buddy told me when I asked if I should tell the boss' son way back about what I saw. As it turned out, keeping quiet was the best decision.