yeah one of my issue with the playthrough that goes "Ian and Lena being faithful to each other since they were both cheated on in previous relationships which is why they both broke up with those cheaters" is that as a player I didn't know Lena's side of the story yet at the time Mike's first scene decision happen.Lena and Axel left their relationship 6 months before the game began, meaning that Axel was unfaithful with Cherry a minimum of 6 months before Lena met Mike.
As to whether Lena tells Ian before or after she sleeps with Mike, the answer is after. She sleeps with Mike before telling Ian about Cherry and also before that moment you decide with Lena if it is a one-time thing or you want to repeat it with Mike, if you decide to repeat it after that same meeting where she tells everything to Ian, Lena contacts with Mike to go to his house to take away his sorrows
We didn't get to know explicitly why exactly she broke up with Axel until after that first Mike club scene.
Which means we as a player in our first playthrough we had to make a decision on taking Mike away from his girlfriend while still not having the information that that exact same behavior is why Lena broke up with Axel and why Lena resents Cherry.
On my subsequent playthrough, there are many times I don't pair Lena with Mike either.
But on that very first blind playthrough, I didn't have enough information about Lena's past relationship yet (especially how much she resent Cherry) to outright not approach Mike.