Hiding a contact. Cheating partner

I have a friend that suspects her husband is cheating on her. Yes, it really is a friend and not myself. One thing she has done is send her husbands contacts to her own phone. What she has found is, the woman that she suspects is seeing her husband is in the contacts sent to her phone, but can’t be found on her husbands phone. So it appears that this contact profile must be hidden on her husbands phone. Is that possible?

Also, my friend, her husband, and the suspected other woman all use WhatsApp. My friend has been keeping a log of when her husband goes online, and when the other woman appears. More often than not, they’re only a few minutes apart. Certainly no more than five minutes apart. So I think you can see why my friend is concerned.

So yes, any help on hiding, and up hiding a contact will help. Also, reading above, do you think my friend has anything to be concerned about.