Unfortunately last playthrough I did Tiffany was already immobilized (sided with Cici at the end of her quest) before I ever even found Ronda, so that option wasn't available. So if you want to fatten Ronda make sure Tiffany is still available (either by siding with Tiffany at the end of the Cici quest, or just making sure you get Ronda the "muscle enhancer" before the end of Cici's quest).