Trust your feelings. It's quite common, for people in a secret relationship, to sometimes bring their spouses along to meetings, in order to hide the relationship. This way they get a "real" reason to be in touch. And if, for example, they call each other and your spouse hears about it, you are all family acquaintances....
You yourself know your husband best. If you suspect, that your husband is in love with the woman in question, then he probably is. Has your husband had other female friends and you therefore do not sense anything shady about them?
I would start the same hobby as your husband and this woman. That way, your husband and this woman cannot so easily agree on secret meetings. I would also recommend checking your husband's cell phone every now and then (once a week). This way you would know right away, if they are doing something forbidden.
Has your husband's behavior changed in any way? Has he become irritable, cold, very affectionate, has he started to pay attention to his appearance, has sex increased / decreased?Everything can change, if your spouse starts an illicit relationship.
Talk to your husband. Ask him to reduce his visits to this woman. Ask him, that you both reduce your visits to this couple anyway. Tell him, that you don't want to have anything to do with them. I hope your husband understands you and you gradually stop seeing this couple.