She can still escape. When the option comes up, you have to not have Rowan tell her he loves her (this will convince her to stay); afterwards, she'll make her escape. I don't remember offhand if you have the option to have Rowan help her escape, but she'll escape even if you don't.
Alexia's trust, mechanically speaking, is represented by her relationship stat (normally invisible in-game) with Rowan. Claiming Helayna causes that stat to go down, but not by a massive amount—she can still have what the game will consider a high relationship stat with Rowan afterwards, depending on how your other decisions have affected that stat. As of now, there aren't a lot of events in the game that increase that stat, but the writers have said that eventually there'll be more; that does mean it's difficult to recover once the stat gets too low.
I will say that revealing the culprit behind Helayna's escape (if it isn't Rowan) can cause another significant hit to Alexia's relationship stat, so keep that in mind (though choosing to have Rowan mete out the punishment himself and then accepting Jezera's help can mitigate that).
O... kay. Good, then.
