On the night out to tell the Givings about their decision not to leave for France in Revolutionary Road, April has sex with Shep in the car. In the scene after that, Frank confesses about his infidelity to April but she remains quiet about her own infidelity. Was this because she genuinely did not love Frank anymore or she wanted to make him think she was perfect and increase the guilt he felt?
First, please notice that there is a difference between those infidelities: Frank has seduced a younger girl and had a passionate romance with her; he has done in consciously out of lust. April on the other hand just let herself be grouped by a friend for a few seconds, only because she wanted to be happy for a moment again, just like she was happy when she was dancing. The former was infidelity, the latter... desperation.
But the main thing is - she doesn't care about Frank's romance:
RANK (CONT'D) I've been with a girl in the city a few times. APRIL Why did you? FRANK (relieved) Baby, I don't know. I think it was a simple case of wanting to be a man again after all that abortion business. Some kind of neurotic, irrational need to prove something. APRIL No. I don't mean why did you have the girl; I mean why did you tell me about it? He is suddenly unsure. APRIL (CONT'D) I mean what's the point? Is it supposed to make me jealous, or something? Is it supposed to make me fall in love with you, or back into bed with you, or what? I mean what am I supposed to say? He tries that same patient smile, but it's not convincing. FRANK Why don't you say what you feel? APRIL I don't feel anything.
At this stage, their relationship is basically dead and April feels as such inside. She doesn't tell Frank about her 5 seconds sex with Shep because for her, it wouldn't change anything: Frank could feel angry and hurt or he could feel better and justified with his romance ("because it is ok if we both did it"), but that wouldn't change the numbness inside her.