I don't have any stats off hand, but a few things might be able to help out. #1 is the framed site listed (well) in the major diretories? ( if so I've seen framed sites rank very well). #2 It would be a good thing to show your client searches related to his keyword search and the percentage of top 10-30 related SERP's that are framed vs the non framed pages.
granted there are more non framed sites that rank well, but maybe many of these sites are non framed for a reason :).