Re: Issues revolving around this one girl.
It may cause some issues, but I'd say she's fair game for you to date if you want... and yes, she does seem to like you.
You can't avoid every girl that a friend might like... it's not like they were dating or she was promised to him or anything. He just liked her, but she doesn't seem to like him, sooooooo... what can you do? Not your fault.
Just realise it may cause some awkwardness, but if he is a real friend, he'll get over it.