Terribly difficult situation, in ethocs and etiquette. One feel sure that the wife eneds to know, to protect herself and their child, even if she finds out anonymously. As you say, he's done this before, and he had no need to do what he did, knowing the risks and not even trying to limit them. I; too, have no respect for cheaters, especially when they choose to risk the life of their spouse.
bel raises another important point --- if you later discovered that she had become HIV positive, how would you feel ? And I see abaolutely nothing whatsoever in his role as "colleague" that entitles him to more consideration than an innocent wife he may be infecting tonight. So he has a right to unprotected selfish sex, but she doesn';t have a right to protected sex ? Can't he even have the gumption to pretend that he has picked up some urinary tract infection, and was advised to use a condom to protect her as a woman's urinary tract is more vulnerable ?
Mona raises yet another option --- to talk it through more with him, to encourage him to tell her ( but he doesn't sound like the sort who would ever do so, and if you started with this approach, he'd suspect it was you if any later anonymous message reached her.