I know there's already been a tonne of responses to this thread, but I cannot believe what I'm reading from most posters about it being fine that she does this!
I also work in the retail sector (currently off work on maternity), and I completely understand your frustration @DawnFawn
The reality of the situation is you have three people on set contracts. These contracts are set at the optimum level of hours where the business is willing to pay and the daily duties will get completed to a high standard. If someone consistently leaves early across the week, while in terms of a wage budget it isn't going to affect you, in terms of store performance and morale, it most definitely will.
No matter what way you look at it, 5 hours of work still needs filled across the morning shifts. Due to the nature of when she arrives and leaves work, it's a 5 hour occupancy that is impossible to fill. You cannot employ someone else to work for one hour a day. It is also incredibly unfair that two other members of staff, disregarding how senior they may be, have to pick up her slack and work harder on their shifts while not earning extra money to compensate for this.
I'm struggling to see how some users aren't getting how annoying and demoralising this is. Imagine you go into work and on your team of 3, you have to work on 9 projects a day. Your shift time allows you to properly focus and work on 3 projects each and complete them to a high standard, without being stressed out. Now, imagine one of your team members starts leaving work early and comes in late. Suddenly, due to this, you and your other team member have 4 projects each to work on a day. You're stressed under the extra workload and you aren't getting any monetary benefit to doing this. All you see is your co-worker stroll in, do less work than you, and exit, leaving you to pick up the slack. It doesn't matter that she's being paid less than you, what matters is she is supposed to be an equal member and she's leaving you in the shit every day. Her behaviour also means it is nigh on impossible for you to ever leave work early because then only one person is working the shift.
@DawnFawn I'm aware that you're going to contact HR and get their take on things which is great, but if at all possible I'd be working to make sure she works her contracted hours or if she does want to work less hours, make sure she's bunching them up and then hire someone else on a PT basis one day a week (annoying but at least you'd have another valuable member of staff in training to act as a replacement if necessary because she doesn't seem reliable- a student or similar would be great for this).
And to those who are sympathising with her travel predicament, seriously? She is an adult, it is up to her to make her own way to work. Many people don't drive, they either learn to drive or take public transport. If the public transport links aren't good in the area, people take Ubers/taxis, or lift share so they get to work before their shift starts. She should not have accepted a job in an area if she could not make her own way to work on time. It's sheer laziness and I'm mortified other managers let her away with it for so long, especially leaving work early! I'd forgive 15 minutes if she stayed an extra 15 minutes, but honestly what is it to me if it takes her longer to get home? That's her problem, not mine- plenty of people commute for hours, it's fairly normal in England so I really don't get why she gets special treatment.