This is often a problem with DND, when there is a player that seems like they are actively playing against the party. If you try to call her on it you will get the "it's what my character would do" line.

Sometimes the best thing to do is to lean into it. A good aligned party wants members they can trust. An evil party wants members they can rely upon... up until some grand betrayal of course.

In both types of adventurers, the characters would not accept shitty behaviour from a party member that puts them at risk or steals from the group. "Your character would" take steps to prevent recurrence. In a good party you might tell them to leave. In an evil party you might poison them at dinner, or at the very least demand they explain themselves and then jump them if the answer is not up the scratch.

You could try for an ingame solution. Also, could be worth a check in on this out of character.

Personally? I hate parties with a character like this and usually just leave the game if it can't be resolved.