Is it illegal to be forced to stay at work when injured?

Hi
I just got a text from my sister ( we always text on her lunch break) telling me she'd slipped at work down a ramp and has injured her hip badly she said it hurts really bad when she walks on it and so told her manager who said that she can just do her work sat down for the rest the day instead and has given her painkillers but that's it she won't let her go home or leave to go to get medical advice and hasn't been seen by a first aider which I thought it was illegal to not been seen after you injurer yourself at work?
Is it illegal that she's been told she can't leave work they need her and have just changed her jobs to sitting down only? Surely she can leave if she needs too?

she defo should be seen by a first aider. tell her to make sure the accident is recorded in the accident book too. she should get a copy or at least take a photo of it.
if she's still in a lot of pain she should leave and go to a&e, her employer can't force her to stay, otherwise they're breaching their duty of care. make sure she documents everything so that she can get a nice personal injury payout when she sues them

Are there any updates? I'm sorry to hear this.