If the beneficiary of a Rep Payee account calls and requests his/her balance information, is there a reason we shouldn't provide it?

I don't really see any justifiable reason why we wouldn't (assuming he/she isn't legally considered incompetent). After all, it is their money. I have tried looking through the SS site but can't really find anything that says we can't.

Is there any reason why we shouldn't provide this information?

You have no dog in this hunt - if they want information, they go to the Rep Payee and then to SSA.

What to do if you have problems with your payee

You and your payee should talk about how much money you receive from Social Security and how to spend it on your needs.
Then talk with your payee about how you want to use your money. If you can’t agree on how to spend your money, or if you believe your payee is misusing or stealing your funds, contact Social Security.

https://www.ssa.gov/pubs/EN-05-10097.pdf