We both work and have shared bank acct and one shared Amex card. Never been an issue until now. We make an equal amount of money.
However she is now squandering marital (family) assets on her affair. Girls' weekend, cellphone used to conduct her affair, possibly gifts for OM or God-knows-what to wear on her "girls weekend" with him.
A family court judge is not going to look kindly on that, and it will be accounted for should you two pursue divorce or legal separation (does your state allow for formal LS?)
At a minimum, YOU need to be keeping an accounting of this, and you also need to lay a "I will not allow our family's assets to be spent on your affair" boundary. We can help you do that.