Not quite necro posting... (been on holiday so...)
The route back to your client is to show both penetration statistics for Flash Player 10 versus HTML 5 compliant browsers. The 99% versus ~15% is a big difference. You might want to point out the ROI of Flex is drastically higher than HTML ( HTML5 especially as it is new ), the fact that HTML 5 isn't set to be completely supported for years... The best thing I've seen from friends who have had similar questions, is when they deliver price estimates for an HTML 5 version versus a Flash version for something moderately complicated. The differences have been significant enough for their clients to drop the idea of HTML 5 very quickly. Gk.