Be there when she needs a helping hand. Show her you care. Help her through her difficulties. Be kind and considerate.
One thing you must not do: Don't run down her current boyfriend in an attempt to win her over. It won't work. You will only end up pushing her further away.
When a girl is in love, it's difficult to get her to think straight. She will be more interested in her emotions than reality.
In fact, it takes an awful amount of bad treatment, disappointments, and hearbreak to get her to see that the guy she's with is all bad for her.
So, if the girl you're interested in is already with another guy and she loves that guy, you will have to be patient . . . more patient than a hyena for her to look your way.
However, if the guy she loves also loves her and treats her good, you may as well just forget her.
Why? Becuase she's not likely to leave him.
The truth is . . . women are more committed in love relationships than men are. Men take a u-turn in a love relationship faster than women do.
Women are more faithful, more enduring, more patient, more hopeful, and more believing in a love relationship than men are.
This means that if she truly loves him and he loves her, and they have plans to take the relationship to the next level, you may as well look elsewhere.
But if there's a slim chance she may still look your way, you may wish to pursue the relationship. The decision is yours to make.
Whatever the case, just be a good friend. And don't be nosy . . . don't make yourself a cog in the wheel of progress of her current relationship. She will hate you if you are. And your dreams of winning her love someday may never come true.