I think you should ask her:
Why should you do it at a 45 degree angle.
What specific muscles are you trying to target.
What is the benefit.
As far as I know, you get the best lat results keeping torso totally upright and using strict form.
Leaning back at 45 degrees will put alot of strain on your lower back and you definately won't be able to do heavy weight without swinging or momentum.
The only times I've seen people lean back are when a really massive guy uses really massive weight and they use their body weight to lean back, in a big swinging/slinging motion.
It's a very loose move with a lot of cheating.
You could get injured too.
I think you should ask your PT to do a lat pulldown in the upright position as heavy as possible with strict form.
Then ask her to use the same weight leaning back at 45 degrees, see what happens...
Seems like she is trying to turn the lat pulldown into a rowing exercise.
There are plenty of safer ways to do rows.