Forget about Trading Standards, they are totally useless and it will take them months before they even bother to read your complaint - never mind doing anything about it.
All she could do is claim of the Mills Insurance for damages. This has nothing to do with Trading Standards.
UK
That depends what contract they had with your partner
In future your partner should mount them, but she also has to be aware that damages happen
to everyone, everywhere
My partner had a concession stall at our local mill shop. She sells framed photographs and the mill take a percentage.
The mill asked her to take her photographs in and the mill would mount them on the wall and display them.
To start with, the pictures weren't mounted until 4days after the agreed day, but worse still, some of the frames were damaged when mounting them and they were then taped back together. She told the manager and all she got told was 'oh'.
Some of the pictures now cannot be sold as the are ruined by the tape and the frames are a write off.
Does she have any rights?