Is Morally OK to do this kind of job? (covering-up AFFAIRS)

Yes, you are rather directly hurting people. Can you live with this? I have no idea.

Recognize that children may be involved. Plus there may be other people involved, not necessarily affair partners. If wife didn't marry suitor #1 in favor of suitor #2, and now you are helping her husband (AKA suitor #2) cheat on her, you are also indirectly hurting suitor #1 by making it easier for the husband to string his wife along.

At least here in the US, divorce comes attached to a rather nasty financial component. Women in particular are often financially devastated by a divorce - and it's usually worse the longer they are in a marriage and not looking for work or socking money away for themselves or getting education. Of course this happens to men as well. Of course divorce is no picnic, but if your actions are directly keeping people in bad marriages, then that is directly financially hitting them.
https://www.brookings.edu/opinions/how-marriage-and-divorce-impact-economic-opportunity/
https://blog.mint.com/planning/the-financial-impact-of-divorce-062012/

Further, if your actions keep people in a bad marriage and they have another child (or they have their first child), then your actions are really impacting their futures.

Want to lie for a living? Then become a private investigator.

If she isn't doing it - someone will.
End of story.

I see private investigator who ruins marriages is the same thing that OP is doing. No?
As it was said before here - cheating don't always ruins marriage - divorces do.