Cheating generally describes various actions designed to subvert rules in order to obtain unfair advantages. This includes acts of bribery, cronyism and nepotism in any situation where individuals are given preference using inappropriate criteria. The rules infringed may be explicit, or they may be from an unwritten code of conduct based on morality, ethics or custom, making the identification of cheating conduct a potentially subjective process.
Muslim is not allowed to cheat or deceive others. If you do an exam for others, you will be claiming that you are that person. This is a type of cheating.
People have a right to privacy, and he has a right to privacy just as you have a right to privacy. There could be any number of reasons, other than cheating, why he wishes to keep his phone private. Sometimes pushing people in these things can lead to greater conflict.
Also, when people communicate via WhatsApp, etc, there is generally an assumption that the conversation is private. (Whether or not it involves cheating) For a third party to read it is also a violation of the other person's privacy, especially if they are talking about sensitive things like personal problems, work problems, legal problems, etc. So it is really not appropriate to read someone else's private conversations. Certainly if I talk to someone on one of these platforms, I don't expect their spouse to be reading it (regardless of their gender).
Similarly, spying is not appropriate. Just as we wouldn't like to be spied on, we should not spy on others.
Might I suggest that if you are having the idea that he is cheating or contemplating divorce, it isn't really about the phone itself, but maybe there are other things that are bringing these thoughts to mind. So maybe it is good to address the current issues between you two (rather than focusing on what if questions). If he seems distant or something else, maybe there is something in the relationship between the two of you that could be addressed, if possible.
If the exam team allows the students to do so, then it is permissible, but if they do not allow it, then it becomes a type of cheating which every Muslim must avoid.