One problem is that you like this person, but you think they like their best friend.

In this case it matters on what kind of personality your crush has.

If they don't like pushy people and don't like much physical contact, but they let their best friend touch them, then yeah they probably like them.

If they are usually cool and cold and they are more kind and outgoing for their best friend, they probably like them.

If your crush is usually out going and kind to everyone and is trying to avoid that person or gets nervous when they are around, then they probably like them.

If they are usually chill and don't care about things much, but get serious and actually care for this person then, yeah, they probably have a crush on their best friend.

[If there are any other personalities you'd like me to do comment them down below]

But if they do like their best friend all you can really do is support them, so that they can depend on you to help them. If they confess and their best friend rejects, then that is probably your time to comfort them, but if they are the kind that are cool and cold or don't like physical contact and pushy people, then it would probably be best to give them some space, but reassure them that you are there for them like one time and then leave them alone for a while, but if they talk to you about it or something then you should try and comfort them.

Maybe that way they will fall for you, but, again, I don't really have many emotions, so yeeeeeeeeeeeeeeeeeeeeeeeeeeeee. derp |:-/