Honesty is the best policy. Just say what you saw and what you think and feel about what you saw during the next counseling session. Please do not hesitate or think that there is some magic way to confront someone who is making your life miserable. The counselor is trained and will help the two of you deal with it.
I agree with Lorie. If your husband is using drugs he is an addict and for an addict the drug is always in first place. And if he's being unfaithful after a separation and been coming to counseling, I believe he's a lost cause. The combination of drugs and and lack of faithfulness is a major hurdle for both of you. I suggest you leave, tell him that when he gets his act together to give you a call.
You really must continue in couselling for your own well being. If you want you can still do counseling with your husband but until he's off drugs you will make very little headway.
When I was in counseling with a significant other the counselor asked each of us what we wanted from our relationship. My main wish was for a monogamous relationship. My significant other said he didn't think he could be monogamous. The counselor then said that he thought the only question left was "who gets the counselor?" It was difficult to let go of the relationship but I realized that was the only healthy choice that I had available. It was obvious we had two different expectations that were incompatible.
We cannot change anyone else. We can only change ourself. I was not willing to accept sharing. I'd already been there and done that with him. It made me miserable. An expectation of monogamy is a reasonable expectation. Expecting him to be monogamous is not reasonable.
Expecting anything from anyone on drugs is unreasonable. They are not able to be committed to anything except the drug.