Depends how close they are too me, if its a best mate (male) then yeah i'd lay in hard. They need to know it and sort it out. If it's a girl best mate, i'd edge around the subject as whatever you say, you're going to be hit or the victim of a huge mood swing.
As for my girl, I dunno, again depends how close / long we'd been together. Tell someone in the opening stages of a relationship would destroy their confidence. Maybe after a year, but I'd suggest something like "Why don't we go to the gym together to keep fit" or sumin. lol