Should the past be the past or play into future?

So I have a good friend who's married to her husband. His friend was over and made mentioned about a business trip to New York three years before they got together. They had already told each other who they had dated and that kibda thing long before they got married but this girl was never mentioned... When the friend of her husband mentioned how he was busy and didn't do the business he was supposed to have been doing she got to questioning who she was and why he never brought her up before when they talked about past exs. She she asked him about her and he never told her nothing other than stop don't ask me again and drop it and everything that doesn't do anything but ignore the question...so she kept saying if it doesn't matter to you now then why won't you tell me and he only responds with stop or the past is the past don't wanna talk about it...so what do y'all think? Please no harsh comments...just wondering what should happen the past stay in the past or should she find out why it is such a touchy subject?