How come you didn’t know she liked him? How come she didn’t know you liked him? Most girls tell each other these things. Is she only an aquaintence? If she is, then it’s ok that you are going out with him, because you would have no way to have known that she liked him too. If she’s your best friend, then I would say that there might be a slight problem because you probably should have known that she liked him. Girls don’t keep that kind of information secret from their best friends, but again, if she’s only an acquaintence, then it’s ok.
If you truly did not know that your friend liked him, then I think it’s ok for you to go out with him and not feel too badly about it, but you need to let her know that you had no idea that she was interested in him, otherwise she will think that you purposely went behind her back to seduce this guy (which would have been really mean). Just tell her that you are sorry that it worked out this way and that you hope she will be ok with it. Then tell her that it’s your goal (whether it is or not) to help her to try to meet another guy that she likes better and then you guys can double date.
If you actually did have some idea that she liked him, then you will have to really be nice to her, and try to convince her that you didn’t know that she liked him. If she finds out that you really did know that she liked him and you persued him anyway, then she will think that you are not a very good friend, fickle, boy crazy and mean. But somehow, I don’t think that that is what happened here. So I think you’re ok. Just be kind to her, she’ll probably be feeling a little blue when you tell her (tell her immediately or she will think that you were hiding the ugly truth from her). Just be nice and sweet and supportive, don’t accuse her of anything. She’ll probably feel like crap for a little while.