I totally agree with the others here. Your daughter's manager had absolutely no right what so ever to open her wage envelope which is her private & confidential information.
Just imagine if there were deductions for attatched county court judgement payments, csa payments or coucil tax arrears payments etc. All of which can be applied to the automated tax deduction system and either payed via tax coding differences or directly applied to the pay slip.
What she should do (depending on the size of the company) is to, speak to HR or the MD and voice complaint. Then if the manager concerned starts kicking off about being reported, she could record conversation via her mobile phone, although this evidence would not be admisable in court, it would clearly vindicate a subsequent further complaint to the MD.
I worked as a shift super visor in a large company and we had a "bully boy" manager who held supervisor's meetings behind locked doors, so that he could insult us and F & curse us without recrimination, or as he thought! I spoke to my MD who had explained what went on. He knew me and also knew that I had been an employer myself, so he asked me to call him on my mobile before going into the next meeting. I did just that, and my phone was on a call to the MD for an hour and a half of not only this manager slagging his supervisors off, but he also graduated to involving the MD in some of his cursings. Needless to say when he came out of the locked room he was met by a red faced MD and the HR management team.
Didn't see him after that, I can't think why!
I would advise your daughter to go to citizens advice where she can be put in touch with an employment law advisor if her initial complaint to HR/MD doesn't get taken seriously.
AS you can see there are plenty of people here who can help with advice and support.
Good luck.
Steve L.