You don’t mention how old she is. There’s nothing in a salary of 12.5k for 45 hours a week to stop her being paid overtime, as far as minimum wage goes you need to tell us how old and also what holiday allowance unpaid lunch breaks etc are involved.
If those 45 hours were split across 6 days with half hour for lunch each day with 6 weeks holiday which would be the minimum that would easily make it minimum wage.
Tips etc can count and bonuses can used to make up minimum wage. If have thought if she’s on minimum wage now she wouldn’t get a real time cut, have the contract checked by a specialist?