Sure, if you have their log-in information.
However, you are setting yourself up to be the villain if you start breaking into other people's accounts, even if you suspect that they are a "cheating spouse." If you are wrong, you will destroy their trust of you, and speaking as someone who was confronted with a receipt for "Niagra" that was dug out of the trash and accused of being unfaithful, you will not get it back. Niagra is a grape variety, by the way, and my accuser and I had shared the bottle a few days before. We didn't share another after that.