She's too manipulative. He needs to put her in her place. He should come clean with Rarity, tell her everything about the photo, Belle's 2nd offer and that he rejected it. Rarity would be furious with Sweetie Belle and Spike would show that she holds nothing over him. Of course, this would end any chance he would have for Rarity but he never had it anyway. He needs a good, stiff shot of maturity. He should then tell Sweetie to reform or stuff it.