He needs a reliable back up as it may not always be practical to get out on time as he could get caught up with all kinds of things like waiting for replacement cover, store searches (we used to get them when I worked in a supermarket, employees were randomly searched as you were leaving to check you hadn't stolen anything), he could get caught up with a customer (common even as going to leave if you're in uniform as they don't necessarily know you aren't on shift anymore) or be in the middle of something he can't just leave (especially with a tight timescale), I'm not sure if Tesco do but some places have a policy where you can't wear uniforms outside of the store so he would have to change or cover up and he would need to get his stuff.
Staff changing rooms aren't always quick to get to and from depending on where they are situated. Ours were right at the back of the store, past the clock in machine, through the warehouse, up a load of stairs - you would then have to go all the way back to the front of the store to leave.
Does he know which area he is working on? Depending on his shift pattern and hours he is legally entitled to some breaks so employers may be loathe to let his substitute breaks for an early finish if he then ends up working over time when he is allowed a break, especially larger employers.