What happens when Arya finds out her imposter is married to Ramsay?

[Lord Littlefinger's Lash]

Bear in mind Arya has often been oblivious and dismissive of the larger context when it comes to other people talking about her or her family.

At Harrenhal her betrothed spent several evenings talking to Arya (disguised as Arry at the time) about how much he was looking forward to their marriage. Arya never realised for a moment the Frey was talking about her.

Arya was equally dismissive when the boy was telling her about Wylla.

Actually Arya was disguised as Nan. But that's a little different since Arya doesn't consider herself a princess anymore than Sansa does since she's never been told that she is one.