I would speak to the manager or the owner. There's no reason for a clerk to refuse to test pool water.

Ultimately, the only chemicals you MUST get from a pool store (like CYA and Polyquat) are available from Internet sources, so once you do your own testing, you can tell them where to go.

Or, better yet, don't tell them. Just stop going and suggest alternatives to your pool-owning friends.

Unless, of course, their superior is smart enough to realize that what they did was VERY bad business and fixes the problem.