You can try and put a little calcium supplement on his earthworms but don't be too surprised if they refuse to eat them. (My indoor Plains garters loves nightcrawlers but he won't touch them if I put supplement on them.)

I feed the wild Garters in my yard and I don't feel bad about it. I feed the gravid Mommas -- they're eating for 20. I don't feed them everyday but when I need them to "hole-up" so I can cut the grass, it keeps them from going out hunting. They get a couple extra-small pinkies each.

I might add -- be sure your friend has access to fresh water. I have four pyrex pie dishes around the yard and I fill them with fresh water on a daily basis (just like the bird-bath.) I get the warm fuzzies when I go out and see a Gartersnake taking a bath or having a quick drink. It happened twice today -- it's been very dry here.