Is it wrong for me to stop paying for this trainer? That isn't a boundary and more of an action she will resent. This is a hard one but I do not see any other option.
I'd like you to start thinking about these types of things in a new way. Instead of asking "How will she respond if I do 'X'? Will she be angry? Livid?? How will her reaction make ME feel?"
and replace it with
"What is THE RIGHT THING TO DO in this situation? What is the thing that God Himself would have me do, if He were standing right in front of me?"
And then steel yourself for any blowback from your wife (and you will get it -- in spades -- on some of this stuff, at least initially) and rest in the fact that you know you did The Right Thing.
If you go by that standard, your decision becomes very simple (not easy, but simple): Why on earth would you PAY for her to meet with her affair partner?
