If you believe that whatever is coming to you cannot be taken away, even as much as a hairsbreadth, and vice versa, you cannot take from anyone else, then this is a non issue. Obviously most of us have not internalized that. But I would believe this is the Torah way of approaching it.
Another point (And this is being said with utmost sensitivity. After ten years of marriage, the shidduchim scenes are still fresh in my mind!) How would you feel, if after being on the market for a number of years, you’re still single, and finally a solid prospect comes up. Everything sounds great, and you would like to go out. The only problem is, that you’re friend went out with this guy, and still has feelings for him. He has no interest in her, and she knows this. But on some level, is still hoping that it may happen. So now you have to say no to this guy because your friend went out with him? There’s nothing to be gained by doing that. Because this guy won’t be waiting around. He’ll just go out with a third girl. Isn’t it insensitive to deny this girl a chance to go out with him? All on the chance of hoping against hope that this guy will not find ANYONE else, and eventually change his mind and come back?