Well, I swore by it for the first two -
With dd1, we had sex Sunday night, and I started cramping within hours. She wasn't born until Tuesday, but I swear the sex is what got everything going. She was born at 39w4d.
With dd2, we had sex at night, and she was born by 3:30 the next afternoon.
HOWEVER! Now I've lost all faith in sex's ability to actually induce, unless baby is already ready...we tried at about 39.5 weeks, and indeed, I got crampy and excited that night...nothing. We tried again a few days later, and nothing happened. Now I'm at 40w5d, and my water broke last night, so we won't be trying again

Either way, it's not going to hurt - if baby is on the brink, maybe it'll send your gf "over" that edge and get labor going...?