How much should she know?

So I need some feedback about a situation and how you lovely ladies think it should be handled... 
A friend of mine recently got married. However he never told/admitted to his wife that he was dating someone at the same time as he dated her. In fact during the relationship he was ready to leave his now wife to be with this other woman but because she ended it he decided to stay with his now wife even tho he admitted he was more in love with the other woman. 
Before they got engaged, I hinted with his now wife that there was things he was keeping from her and since she is the highly jealous type that she should ask him about it. 
When I talked to him, he said before they married that he wouldn't tell her until maybe 20yrs after they were married. Because he said if he told her before then that she would break off the engagement/relationship. 
Fast forward now almost a year later when they are on vacation and he takes her to the same country and same village that this other woman lives in, where he met her. His wife had no idea the real reason why he chose to go visit there. 
Should I do anything? If you were the new young wife would you want to know? Keep in mind there's almost a 10yr age gap between them and she was 18 when they married. He's very manipulative and controlling of her and obviously doesn't tell her the truth.