Funnily enough, tools are best monetised as SEO enhancers. His tool would have incredible page rank, and by linking to a "Made with love by congressblog.com" from that tool (and all the others he has made) and then populating congressblog.com with lots of content about like, congress, he could monetise THAT site with ads. He didn't have to ruin the calculator.
EDIT: an alternative monetisation source if the OP didn't want to create a bunch of content would be affiliate links.
EDIT: an alternative monetisation source if the OP didn't want to create a bunch of content would be affiliate links.