is my ex going to get himself hurt?

[Rose]

The man I was seeing fairly seriously, he had even said he wanted to be my soulmate and saw us growing old together, for almost a year broke up with me recently as we were in an open relationship (because that's what he wanted, to be able to see us both) and the other woman he was seeing had been married for 11 years and still was when we started dating and left her husband because she was afraid of losing my ex to me. So she left her husband and moved him out and my ex in the same day and they have been monogamous ever since. She and my ex had been open on and off for about 3 years...I just want to know if this is something that seems like it will work because I am worried about him getting hurt, they seem very happy playing house now and she posts about their relationship on facebook atleast 3 or 4 times a day and since we are all friends on facebook it pops up in my news feed all the time, i have tried to keep them out but since we have so many friends in common it comes up anyways. He says they are in love but if that's true why didn't she leave her husband 3 years ago? doesn't that seem more like infatuation? I just love him and I worry about him...

do you think this relationship will be lasting or is she bound to go back to her husband or someone else for that matter?