Show them this:
http://www.youtube.com/watch?v=Rk4qgQdp2UA
If you mention it once, good. If you mention it twice, ok, you're about this. If you mention it 3 times, ok ... getting less helpful. By the 4th, 5th, 18th time you mention it, it's not really helping. And if you overdo it it does hurt.
I've found (and YMMV) that keyword consistency is more important than density. If my article is about flowers and I mention roses and daisies, sunflowers and tulips - that's more important than saying "I like flowers because flowers smell nice and flowers are pretty flowers and you can give your wife flowers on Valentine's Day, a flowers holiday."
Also as far as consistency, use the main keywords in the title & description but don't spam them in there 3 or 4 times. Then use those keywords and the related set of keywords (types of flowers, potting, plant, garden) on the page. That's more helpful, I think.
(I'd suggest you could show your client a forum topic where a bunch of professional SEOs say keyword density doesn't matter but I'd really avoid using the word "idiot" when talking about the people who pay you if you want them to continue to do so.)