Is everyone in mid life crisis a midlife cheater?

No. This isn't really the forum to talk about pop psychology, but i appreciate your asking. There is no understanding people, but it's natural at any age to change interests and want to make life more special in some way. You do yourself a disservice if you tell yourself that it means infidelity, whether it's your partner, or a friend, or a family member. It may be a very positive move instead.