As one of the best chat apps, WhatsApp is often used as a means of communication, especially for long distance relationship (LDR) couples or those who cannot be together all the time.
In a relationship, there will definitely be suspicion of a partner. Well, to minimize unfounded suspicions, there is nothing wrong with knowing the content of the whtsp chat.
Actually tapping is not ethical in a relationship. Therefore, you must ask permission first, but make sure that your suspicions have clear and real evidence.