Contacts are an area that icloud is well known for putting back old data from an older sync, especially if there’s an older backup that gets restored, etc. Restoring from an older backup is EXACTLY what you’d do if trying to get an older password, as well.
I think odd/old contact data is maybe the LEAST concerning thing you could find on a device, especially an unlocked device left in an easily accessible shared location.
Obviously, we have no insight into your larger situation, but I’ll agree with others who have said that suspicion and playing ‘gotcha’ games is at least as corrosive as cheating. People tend to pursue ‘gotchas’ as a way to justify their own upset and push away accountability for what they already know, which is that their relationship is in trouble.
If everything else is fantastic, then don’t worry about it and get on with your fantastic life.
If there’s other stuff that’s a problem, then work on fixing that or break up and move on.
Cheating is most often a symptom of a person or relationship in serious trouble, rather than the starting point for the trouble.