I'm not sure if his friend is being manipulative so much as he doesn't think a relationship with you is worth causing issues in his friendship. I've been in that situation before. A new relationship is often not worth risking a long term friendship. I've walked away too.
This is not to say you aren't a great girl and he is missing out big time. But it is what it is. He's not THAT into it that he'll risk having issues with his friend or want to deal with the drama of his buddy crushing on his girlfriend. That's just the way it is.
I don't think there is anything you can do or should do. It's just the way this scenario worked out.
good luck and you seem to have no problem with guys, so keep looking for one that you'd have an easier relationship with.
Hmmm.....two guy friends who like the same girl (you).
Bonds between two good male friends can be extremely loyal and strong. I wouldn't recommend trying to come between two good friends.
This happened to me YEARS ago; was 19 years old. I couldn't bring myself to cause issues between two good friends, so I thought more about their friendship then myself. Plus, I wasn't up for all the drama, so.....it was ended.
DO NOT keep trying and end this. These situations are always drama-filled and nothing but headaches. It will not be worth it in the end.