The wife having an extra marital affair does not invalidate your Nikah. The extra marital affair is Haraam and a major sin which incurs the wrath and curse of Allah.
We suggest you speak to your wife and clarify the issue with him. However, you should use a diplomatic approach rather than an accusative one. You should try to consult the seniors of both (yours and his) families in trying to resolve your predicament.
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai