I hate to be practical but he may not be practicing protected sex and the partner could get a STD which can cause adverse medical conditions, some resulting in death.
Now for the moral aspect. You state he has participated in similar behaviour prior. Just my view but I think extra marital affairs are a strong breech of trust and are next to impossible to resurrect. If he is inclined to want his cake and eat it then he should make the break from the marriage. I have strong feelings on this issue and I fail to see any excuse for having sex whilst you're in a committed relationship.
As Dino suggested, if someone must then seek the services of a brothel, that's why they're there for. I suppose most find the thrill of a work colleague more enticing.
I agree with Omorrisey, send a letter. Watching a committed friend being deceived by a sleaze bag is not on. As to the children, it's difficult but using them as a human shield can be harder.
