Sounds like a made up story, but if it's real...she should tell him, and give him the option of staying. He has a right to know what he's involved with...
Gee, basing love on a lie. Hmm, what could possibly go wrong with that, eh?
Everyone involved deserves to know the truth, but since it has gone on for this long, I'd be inclined to carry on the charade for a few more years until the kids were adults. Then sit everyone down and spill the beans.... then duck and cover... as it will not go well.
Sounds like a made up story, but if it's real...she should tell him, and give him the option of staying. He has a right to know what he's involved with...