If he has changed for real, then it is wrong to tell her,
or anyone for that matter about his past. I know many happily married couples, some of them already marrying off their own kids,
that the man had a past which he left for good. Men especially when they’re teens go through a little rough patch till they find their calm. I know a case where they asked a family friend of theirs about the boy before they made the L’chaim and this friend of kallahs family was aware of the boy’s past.
They didn’t say anything other than he was a super nice guy, which he was at the time. Now B”H they are very happily married. He is still very much a Mentch and a very warm person.
However If he’s currently a person that does things that if
known to her she would not be interested, then he must tell her before. If that is not an option, You must ask a sheilah to an actual Rov, If you can and how to divulge this information.
Do Not ask virtual Rov, or collective body of wannabe Poskim.
Reb Moshe Paskens to a woman that she must tell her future husband
about her past i’m not sure to what extent but that it is behind her. So I’m sure that this guy if he’s still not behaving properly
he must be honest.