In my case, the OW was a good-time party girl and pushed my H to start drinking again. She didn't know she was opening Pandora's Box.
Once H resumed drinking, his life was a series of DUIs, arrests, fights (he even hit the OW and was charged w/Domestic Abuse), lost his job, and all his money.
The OW has complained to my MIL on several occasions that she wished H would give up drinking!!
Believe me, these relationships between MLC spouses and the OPs spiral down quickly. In my case, my H sees the damage he has done but he's too stubborn to admit that he screwed up his life so badly.