They've been good tenants for 2 years, been on this advanced rent arrangement the whole time and only now are asking for a one time car repair, one month of six prepaid months to be returned? Is that right? Is the prepaid arrangement part of the written lease?
I think the rent refund is a dangerous precedent, and as described, likely the beginning of the end. You may also run into problems in court, if it comes to that, of not offering this type of arrangement to other tenants as part of your normal course of business. Equal housing laws really dislike special treatement for any single tenant.
I'd take a page from Nancy Reagan, "Just say no." Too late for the agreement, but, if they asked you to repair their damage with the security deposit they have with you, would you agree to that? Technically, if you have this arrangement written into the lease agreement, doesn't your acceptance of this returning the prepaid rent put them in violation of the agreement, and your voluntary acceptance of this void that clause all together? Not something I'd want to test in court.
At the same time, a good tenant of two years is worth keeping, for sure, but I'd find another way. Realizing that the need for the money stems from car trouble, I'm going to guess the car is paid for (bad credit means no loan), and that the repairs don't really equal 1 months rent. Perhaps suggest they pawn the title for a couple months, and if the loan shark rates are too much, consider a non-landlord solution where you personally loan them the money, have them sign a promisory note for a nominal interest rate, and they sign and you hold the title. Make sure you pay the repair shop directly, and not pay them. Make the whole thing business like and separate from the LL/Tenant relationship. If you have to be the bank here, don't mess with the rent, be a secured private lender. Then, you'll keep the lease clauses in place and paid, hopefully gain a little goodwill with the tenant and it'll be another two years of no vacancy and good tenants before they approach you again (and they will).
Good luck, and let us know how it goes.
Updated: 02:49AM, 10/07/2009
Oh, and I knew a LL that once co-signed with a tenant on a short term unsecured loan from a bank. Don't know why I didn't remember that when I wrote my reply, but, that helped the tenant at no cost to the LL.