Maybe you could bring up the subject of your need for transparency from your wife, considering her affair? Not so much that you need to talk about the affair itself -- just your need for transparency now.
However, it sounds like your MC might not have your back on this subject, in which case I'd get another marriage counselor. Without someone specifically trained in dealing with infidelity, including treating it as an addiction (separating the addict from the source of their addiction, via no-contact letter and full transparency), I personally think the MCing will be fruitless.
