First of all, they shouldn't eat anything with fur on it. Their systems just aren't built to digest it and it can cause intestinal blockages. If you're feeding an animal anything live that has teeth (this applies to any reptile) you should supervise until the animal is dead. A bite can do serious damage to the snake. Finally, wild garters rarely eat mice. They're primarily fish/worm/amphibian eaters. Plus, they run pretty heavily on instinct. If you're going to be releasing it in a couple months anyway, feeding mice probably won't affect his wild eating habits at all.