but she's pissed that he fucked someone else?!?!
Lena is very protective over Holly and doesn't believe that she can take it being used by man, hence she either drops Ian or leaves him so that he and Holly get together, if he fucked Holly. Though it is possible that it ends in a throuple in which all threee love each other.
A similar case is when Minerva ends her affair with Ian if he dates Holly, because she believes Holly wouldn't take it well. Once again, Holly is the only girl where Minerva cares.
Louise also doesn't sent a text if Ian is with Holly. He can be together with every other girl and she still texts him to flirt with him. This could be sloppy coding to avoid messing with Holly, or Louise knows Ian is taking with her and once again doesn't want to interfer, while she has less issues contacting him if he is with Lena (currently a questionable plot point, at least Ian comments on it and can back out because of that).
Overall, its impossible for Ian to cheat on Holly in the long run, he automatically ends their relationship on his own if he still has feelings for other people, because he doesn't believe he can make her happy, then.
Hollys special threatment is sort of consistent in that people around her correctly assume that Holly can't take getting cheated on as well as say Lena, who is a used to it and tougher. Still, EvaKiss bias and special plans toward Holly are noticable and it can understandibly seen as inconsisten since Ian and Lena are behaving different with other people.
While Ian and Lena can cheat a lot, at least in Ian case it starts to have consequences and he is forced to decide between ending love relationships or stop cheating with some exceptions. In Lenas case, it is questionable how she can still fuck 10+ people in one run and don't commit to anyone aside Ian. Supposedly its the nature of her relationship that she isn't serious with most of them, but she really should start commiting and ending other relationships. She does it with Ian, but there are still many flings that can still be go on and it can be seen as a writing inconsisty, either voluntarily by Eva, or involuntarily if the many choices and paths just overwhelmed them.