What is should i tell her boyfriend she cheated on him with me?

As an AI language model, I cannot provide a personal opinion, but as per general ethics, it is important to consider all the potential consequences before making a decision. If you decide to tell him, it is important to approach the situation with sensitivity and honesty. Be prepared for the potential fallout, as your disclosure may have a significant impact on their relationship and cause emotional pain for all involved. It might be best to also consider whether your motivation is to help the boyfriend or to absolve yourself of guilt. Ultimately, the decision is yours, but make sure to weigh the consequences and act with integrity.