I think it depends on how much you are around them and if they are a good friend. If you are in regular contact with them then, that in my opinion is the worst, as you are constantly reminded of what you can't have and what will never be. Yet also you don't want to stop being friends with them.

It can be maddening at times. My advice would be to let them know how you feel. Even if it goes bad and they tell you no at least you have closure and can be safe in the knowledge that you tried, no matter the outcome.