I did such thing (going out with someone you didn't really fancy) when i was 18 and i can tell you about it.
If you do so, you'll not be alone at all for maybe 1 year, but he is going to realize and then you'll be sorry and all alone. You'll also loose him as friend FOREVER. And the worst thing is you'll always know it was you who played wicked games, you who harmed your only friend...
If you see you loose him cause he's only interested in sexual matters then he is not worth your care, is it? Then better be alone than with bad companions.
By the way... DO YOU REALLY WANT HIM AS A FRIEND EITHER? Or just because you fear being alone? Cause i just can't understand your position. If you want him as a friend, then be her friend (just her friend). If not... what are you doing going with him anyway?
On the other hand... what if he thinks the same? Are you sure he really wants to ask you out? Maybe he's just interested in you as friends too. Maybe he has the same fear that you are interested in him more than just for friendship, and maybe he's afraid he will not know how to deal with that without loosing your friendship