No no no no Tifa is a horrible friend for wanting to seduce Cloud. If he truly loved Aerith he'd hold out no matter what, because when you are in love you loose all interest in everyone else. If he was actually in love with Aerith he would have no desire to be with Tifa. Therefore, the Highwind scene consisted of nothing but fully-clothed spooning, because Cloud is a nice guy so he just thought letting Tifa sleep on his shoulder was enough. Notice, though, that Cloud is turned away from her. He was obviously remaining celibate, but just letting Tifa feel wanted even when she wasn't.