I know it is a scary thing to do, but if you love your friend, assuming you are both single, why not tell them? Because of friendship? Nah. 20 years later, are you sure they will be still on your side? (Unless you are neighbors lol) People change, they will have a family, or move somewhere, so how is your friendship then, online? In short, what am I saying is, friendship will still be ruined no matter what. Tell them and be brave, who knows what will happen? Instead of telling them while they are in a relationship, and ruining it.