If you were feeling overwhelmingly bad about it, the client could always have the tests done out of his own pocket and you could reimburse him if it came out to be anything that might fault you or the other breeder, which it doesn't sound like it was. Assist feeding a snake for their very first meal in your care even though they've been good eaters before doesn't sound like good husbandry practices to me. I may make small mistakes in husbandry every so often, and as owners I think people should recognize when they made an error instead of automatically blaming whoever they bought it from. I would certainly ask questions if a snake I had for only a month died, but demanding compensation for something that may actually be my fault would only make me look stupid.