How do you know it's a curse? Maybe your friend is just naive and young? My sister was in love with a guy who wasn't nice at all. He never did magick, didn't believe in magick, he just knew what to say to make her come back. I wish i could say she saw sense, but eventually he left her. She's older and wiser now, but at the time, you're 17 and think it's love.
as Essy said, magick doesn't work that way. A love spell makes a person want to be with the caster, but subconsciously they are fighting against the spell. If the person is a "bad" person, you're friend would of broken free long ago. Curses also don't work this way. They bring misfortune, they do not cause the person to blindly obey. Bindings are a third option, but these spells do not cause the person to love the caster, it causes the two to share a connection so they will keep bumping into each other. Hence Handfastings when you wish to marry.
Talk to your friend, but you should be there for her. Sometimes being told some one is bad causes the person to take a "I'll show you" attitude and stick with them much longer than they would like.