Question

Can you tell my friend this is right so he can stop saying it is wrong

Answer 1

Your graph is correct.
You start with point (0, 40). On day 0, you have 40 T-shirts.
Each day that passes, you have 5 shirts fewer. (1, 35) means on day 1, you have 35 shirts left. Each day that passes, there are 5 fewer shirts. On the 8th day, there are no shirts left. That is point (8, 0).

Good job!