Do you think it is important and/or necessary to inform you SO of your previous relationships and relations?

I was thinking about this after a discussion with a friend. Our friend’s situation is they had a summer fling with someone. It ended awkwardly but they are still good friends and on great terms with the fling. Then a year later they started dating the fling’s roommate. The roommate has no idea his roommate ever had a fling with his now girlfriend. Is that something our friend should tell her boyfriend? Or since it is in the past, should it stay there?