Is this allowed??? part time work??

You should not go in and speak to her boss. And as a 16 year old, she ought to have the energy to stick out 5 hours of work without a break. She's entitled to 15 min. When you're working in a busy place, there is no guarantee that you will get that 15 min. after 4 & 1/2 hours on the dot. No one can just drop what they're doing and run off for a break. You take breaks when they're convenient.