Just a quick enquiry please. How easy would it

Hello,
Just a quick enquiry please.

How easy would it be practically to connect the login on WordPress to another php /mysql driven script on another URL. This is so users do not have to log in twice.

The Sync plugin won't work in this case I suspect strongly on a non WP site.

I just wanted to check before I hired a freelancer to do this. If it is a complete monsters of a job or just not possible then I won't bother.

So on a ease sliding coding level of 1-10; where would this be please?

regards