Employee discounts are like affinity discounts. You get the discount if you are affiliated with the employer and if you leave you should not be eligible On the other hand, did you specify that in the employment contract or rental contract? Did you specifically say "Rental discounts of 20% are available only as long as you are in full time employment (or work more than 25 hours a week on the average...). If you leave our employment for any reason, your discount will be withdrawn.

Also rental contracts should be separate from employment contracts and this clause should be ideally in the rental contract. If you have not explicitly told her (in writing) that this discount is valid only till she is employed, she is within her rights to assume it is valid till she leaves the apartment. So it is a contractual issue. If you do not have an explicit policy for discounts (say in the employee handbook and the rental contract), and you have not told her it will go up, you are TA