1- If one does not have evidence, one should ignore it and ask Allah for the best. It’s not healthy to constantly doubt one’s spouse.
2- If one has evidence, one should confront the partner in a calm and effective manner. One tells the partner: I have become aware of your other relationship. Please explain why you did that? How can we fix this because I cannot continue like this? Is there something I can do to help with this situation? If the partner sees you reacting properly and wisely, chances are they’ll try to fix it.
3- If one’s wife cheated and commit this sin, one can still live with her Islamically, but one can divorce her and it would be ok to divorce her. One can peacefully end the marriage and move on.