Hey reframe - I feel your pain. this is not easy to navigate.
One thing - the advice and feedback from the DB Coaches is generally a bit more softer than what you will find on these boards. I say this from my experience and also others. The coaching session was extremely helpful for me, but I made changes to my approach based on the sitch and how I was feeling.
Sandi's rules talks about being pleasant, polite, and acting 'as if'. None of that is beta behaviour, but it's about you carrying yourself with confidence and self-respect. This is very had to pull off in the beginning authentically as the LBS is still reeling from the loss and is feeling out of control. That's why no contact/going dark is truly for the LBS to get some stability and get some solid ground beneath their feet.
If there was no A, I would lean towards being a supportive friend, but even then I am hesitant to full co-sign that. I don't think it makes you look strong and confident.
In the case of a confirmed A, and how your W revealed to you the A, that is just complete and utter disrespect towards you. I am sorry, but at this point being a supportive friend is not going to work for you.
As you also said, she messed up and you don't know if you can ever trust her again. So, why are you trying to be a supportive friend? what is there to gain in that? - maybe she'll turn around? You have no idea and you can't 'nice' her back. She'll lose even more respect for you.
I get you're feeling lost and hopeless right now. Been there man and it's an awful place to be. The only thing you should be doing right now is focusing on your emotional and mental wellbeing, and not worrying about her. Not worrying about how your actions might cause this or that. Not analyzing every little move and conversation with her. None of that.
Realize your MR is dead. She's gone. If she comes back, you will have to rebuild this MR and it's not going back to status quo.
Grieve and feel and process your emotions. Go see an IC if you believe you need it. Then get up and start GAL, start focusing on yourself and your goals. Make yourself the prize that only a fool would walk away from.
DB might not save your MR, but it will save you. Just trust the process - it's hard to do in the beginning because it's counter intuitive. But trust me from experience, the process is good and meant for your protection and wellbeing.
joejoe gave you good notes on how to act during the meet for taxes. Follow that.
before you act, come and talk to us here. don't act out of emotion. you're in a place right now where you need support and feedback. We're here for you. if you don't know what to do, do nothing.