Here's another site that I contribue to:

Spay/Neuter Info Resources

(Scroll to the bottom of the page for further resources.)

I think the best lesson about the overpopulation crisis is to take that person to a shelter and talk to the staff. They will show her how many animals are put to death DAILY because of people like her, who believe non-spayed animals is not a big deal!

Good luck in educating these people! Education is the key.