I'm sorry that you've found yourself in this situation. This news article from 2014
mentions the issue.
Even where marijuana is legal, the chemical content is not controlled, making it a problem for medical study. As a result, less is know about how it affects you in combination with other medications. You can assume that it's an outlier when they look at your medication list and consider what you'd need to take before and during a transplant.
I don't know if there's a rule about THC in your system during transplant preparations. You'll have to ask. Don't worry about bringing up the subject; it's always a mistake to keep information from your physician. If they give you an OK, then you don't need to continue worrying about being rejected for transplant.
If they confirm what your friend said, then you'll have to make a choice. You don't want your anxiety and PTSD to be left untreated, so given that you're between a rock and a hard place I think you should reconsider your stance on other drugs. If the medical team recommends certain prescriptions, even from big pharma, then that gives you a way to stay within the transplant guidelines. And perhaps they'll have suggestions other than big pharma drugs, so express your preference to avoid them.
It's unfortunate that it can take several tries to find the right medication to treat anxiety. Different medications work for different people. So the sooner you start the better. Good luck.