If your girlfriend wants to hassle to get that letter, she should first ask whether the branch she works for is owned by Nando's or is a franchise operation. (That might be self-evident from her pay slip anyway. If it's a managed branch, rather than a franchise, her pay slip will probably be on official Nando's paperwork. If it's not, it's probably a franchise).
If the branch is franchised, she's not actually working for Nando's, but for the franchise holder (who is, presumably, her boss at work). In which case, all she can do is to continue hassling him, possibly by turning up at the branch, during quiet periods when she's not working, and making it clear that she's not leaving until she gets her letter.
If the branch is managed, she needs to go over her boss's head and speak to someone in Nando's HR department. The company handbook should contain the appropriate details. Otherwise try phoning 0800 975 8181 and asking to speak to the HR manager.
Chris
PS: Thanks for your replies to my question in 'Food' ;-)