I would suggest she goes back to them to complain, not to you. If they do not take her seriously then I would imagine the next thing for her to do is contact Trading Standards.
Just remember that although we feel sorry for clients that have been ripped off by other technicians it is not always the case. Sometimes people just like to complain and if they complain to you about someone else they will probably do the same behind your back.
If she has had a bad experience all you can do is help her get her nails back to good condition, don't get involved with the drama. Point her in the right direction and don't say anything bad about the other salon to her. Who knows she might go back there gain and if they know you are saying bad things about them you will be the one to suffer, hope this makes sense, I know what I am trying to say hope it comes across in the right way.
The reason I am saying this is that I did a lovely manicure and pedicure months ago, gel extensions and gel polish on toes, the girl then went to Mauritius for a fortnight and 2 weeks after returning sent me a text saying her nails were a disaster wanted her money back etc etc. It turns out she spent loads of time in the hot tub, the swimming pool, on the beach, doing adventure sports and covering herself in DEET. All against my aftercare advice. She is usually a nail biter and was not happy that I would not do really long nails for her, I did a manageable length. She refused to let me see them and I refused to refund. Anyway she then sent me texts telling me that she has showed them to other salons and nail technicians and they have told her that it was a bad job. My insurance company backed me up and told her to swivel. Of course they would look in a state after all the bashing, picking, chemicals and biting and no infills a whole month later. I was fuming that other technicians were so quick to jump on the bandwagon without knowning the full story.Clients often only tell you what they want you to know. Many really Good Nail professionals use e-files, many really good technicians are Asian too so it might not be good to just assume the worst.