The difference with Louise is that Ian is involved. Ian has stated that one of his fantasies is an FFM threesome and Ian and Lena have agreed to experiment together. Lena is essentially acting according to this agreement.?
Mark is the Asian dude at the club, I think you're talking about Mike. Or maybe I'm forgetting something.
Even so, my point is that Lena having sex with Louise is cheating. Your counter was that Lena isn't romantically attracted to Louise. So another example of Lena having a sexual encounter with someone she does not have romantic feelings for is with Mark in chapter 10.
So either her blowing him in the club bathroom while dating Ian is not cheating to you or the romantic feelings thing was irrelevant.
If this is the first time Lena has been with Louise since getting together with Ian then (IMO) it should not count as cheating.