Should I Tell My Friend Before It Gets Serious

A few years ago, I started seeing a guy. This relationship was casual and not very serious,therefore,no o many peop knew about the relationship between the two of us. Recently, a very close friend of mine started seeing the same guy. They are in the beginning stages of dating, or whatever they are doing. I do not know if they plan to establish a relationship or not, but a part of me feel the need to tell her that the guy and I have had dealings. Another part of me wants to keep quiet and not cause any interference between the pair. I have never been emotionally attached to the guy, and still speak to him when I see him. My thing is, I do not want to have to keep a secrect like this from my friend if she and he decide to establish something. Please give advice on how to approach this situation delicately. Sincerely Yours, MissJustWannaBeAGoodFriend