While Mac is supported, many Mac users will be even less likely to have it installed or ever install it. Majority of schools and probably health facilities too will not have SilverLight installed and a large proportion of home users will either not have it installed (and not want to hassle with installing it) or will be on a Mac. Sorry but SilverLight is just not the best option for reaching his client's market and actually making it easy for them.
But his client won't be thanking him in 5 years time. So bang goes a large majority of the mobile market - only one of the most important markets and the major one that is growing fast in the education sector - especially for kids/teenagers. I haven't been able to reliably (if at all) get SilverLight on my Android phone either (Samsung Galaxy S2) and that's a relatively new device. It would be better if the OP programmed an HTML5 web app that will last probably for the next 15 years, rather than a SilverLight one that will break upon the next major update or when MS decide to stop supporting some browser or when another one pops up. Using the standard rather than always trying to create new ones is better. I see from your profile that you favour SilverLight a lot.
That, and SilverLight won't last - did you believe Flash would last forever? Probably. Controlling everything is not always a good thing. I'm saying use the standard rather than the hack.
