I would not advise anyone to use the court's resources for such a small claim of £51 - it is hardly worth their while. The court issue fee would be £35 just to get the claim started.
She also left the salon "happy" and prompted the service on social media too - they even replied to say they were happy with the service. So their consumer rights have not been breached in respect of the service itself. All they can allege is that the service was not "as described" in terms of price.
My view is there are no unfair terms in the agreement either. If she wins she can only recover interest at 8% and the court fees (the issue fee and a hearing fee though I doubt there would be a hearing for the claim with it being so modest).
Diana also sent a text message which can be used as proof and the message infers that the price varies according to how much product is used.
I would not be inclined to refund the £51 as I don't see that you have breached her consumer rights.
If they do sue, you will receive court documents (a response pack) which you must complete and return to the court. It is a tick box exercise for the most part and there is a short section to write a defence which is easy enough though please feel free to come back to this site if you need any more help. A claim will also take 9-12 months to be decided at court. If you lost then you would get 14 days to pay the judgment before the claimant can enforce the order, and 30 days to pay in full before it is registered with credit agencies. The claimant cannot recover legal costs if they win, in a claim under £10K (a small claim), all they can claim are the court fees and interest.
If the claim has no merit then you have an option (after your defence is filed) to apply to strike the claim out. The court will consider an application if the claim has no merit, or is misconceived. The application costs £255 but this is recoverable if your application succeeds. If you are on a low income, have low savings or in receipt of benefits then you can ask the court to waive the court fee. If you won the application, the claim is struck out.
The hearing (if the case gets that far and if the court needs one to be able to make a decision) is likely to be held remotely, it's you, a district judge (who is a practising solicitor or barrister) and someone from the claimant company. The Judge decides and if you lost, you get 14 days to pay the sum. If paid in full within 30 days then nothing goes on your credit record. But certainly I think you would succeed in a defence, just to be clear.
I can assist you going forwards if it gets to the point they issue a claim.