Yes he has right to withhold information as he wants. BUT what kinda friend is he if he does this?
Like i said, it depends on his perception of what would be consequences of revealing it, and his intentions.
Jeremy notes that Ian spent a literal year depressed and moping over Gillian, and only recently started to move on. It's not unreasonable for him to think that if Ian learnt about Gillian, he could relapse in his earlier behavior. And that withholding this info he's helping his friend to stay on his present course.
To use an exaggerated example, if Jeremy suspected Ian might literally off himself if he learns about Gillian being in town and still fucking the guy she's betrayed him with, would he be "a good friend" in your eyes by immediately running to Ian with this info, without care about the consequences?
they grew together, been together from college(i believe) and than goes on telling wade "I don't steal in friend house" bullshit and guess what he does with Lena?
He falls to the player's magical "i get what i want" button. If the player chooses to use it. Otherwise, he keeps his distance.
If you want to hold it against him, how do you feel about reverse situation, which is Ian knowingly hitting on Alison and cock-blocking Jeremy from her, even though it is Jeremy who first brings up possibility of getting together with her, and you know it is something he wants? And doing it not even because the player finds Alison their girl of choice, but purely to get one over Jeremy? Because i don't really see people in this thread feeling like "bad friends" for doing it to him. Quite the opposite, they brag about it and relish this opportunity.
Imagine if Jeremy took this attitude with Lena? Mocking Ian to his face for getting it on with her? This thread would be cinders. But the opposite seems to be not just a fair game, but a local sport.