Just wondering if anyone has a solution for the following:

I want to work with a partner website (not my own), that will give a discount on their product to users of my website.
I would like them, as part of their registration for the deal, to validate that the user is also a user on my site.

I could simply give them a file containing usernames that they could validate against, but with many users joining my site daily, and a user base of over 5000, I am looking for a solution whereby, once the username is entered on the partner site, and the 'validate username' button is pressed, that it looks ups the username field from my database, and sends back a yes/no or 1/0 type response.
If it's a yes, the registration may proceed.

I'm not looking for the solution on their end, just a solution for my end.

I want a method that they can have access to query the usernames field on my joomla database, and get a yes/no response.

Can jBackend do this easily?
I'd like to know there is an easy solution before buying this plugin.

(I'm not an SQL programmer, or anything like it, so please go easy on me!!)

Thanks,
Anton