Dear kel,

This is always such a loaded question. There are a number of things to consider. 1. Are they in a committed and monogamous relationship? If not then this isn't any of your business. 2. If you have so much proof how likely is it that the person being cheated on doesn't know? Often they are deciding to stay for reasons you might not understand. And if this is the case then they're not ready to do anything about it and won't take kindly to having it pointed out to them by you. 3. Are you prepared for the person being cheated on to stop being your friend? This happens often because of the above reason. 4. If they truly don't know about the cheating then certainly the chance of being given an STI or STD is hard to ignore. An option for you is to talk to the person who is 'cheating' and tell them you know about their cheating and if they don't tell their partner you will.

Coach Christine Expert