Your friend has a job at David’s Tea and she gets a 50% off discount on everything in the store. You ask her to give you her discount because you want to buy tea and she says yes. Are you allowed to use the discount? Is it considered stealing? Are you stealing from David’s Tea or your friend that is letting you use her discount?
Yes this is a form of stealing and the discount may not be shared.
When a store provides special discounts to certain people [such as an employee discount], they are intended for those people alone. Someone who abuses this privilege to buy for others who are not entitled to the discount violates the prohibition of theft, unless explicit permission is received, See Pischei Choshen, Gneivah 1:1.