How would you determine if she passed the test?

I think it's your therapy and you're the boss but it seems hostile to start with a test or with the assumption that new t is stupid. It's not her fault that your old t is leaving in the middle of such difficult stuff. To me, it makes sense to "test" a T by starting slowly and paying attention to how you feel around them, how they react to what you say and letting trust build gradually. Did you have a specific role in mind for your friend?

If you feel it would be helpful to have a friend along for moral support maybe they could wait for you in the waiting room. It just seems like therapy is so intimate and private... I'm not sure how a friend belongs there. But if you do, you're the boss like I said!