when you have the dinner at rachels after the big argument scene and the fight and you are downstairs and free to move about talk to rachels mom and find the option that asks to help her out aka help her clean the mess she will ask you to get the broom out of the pantry next to the fridge the hidden shirt is in a box next to the broom you can pick it up and it will be available to change into the next time you are by a place you can change your outfit.
hope this helps
i dont see anyone rocking this shirt so