I used to work in liability claims, and in order for compensation to be forthcoming, your friend has to put in a formal claim to the supermarket. Did the accident get entered in the store's accident book? This would normally be the first step, as it'e evidence that the accident happened. Fid he get the names of the people on customer service who he reported it to?
What he needs to do is to write a letter, stating exactly what happened, date and time, and referring also to the phone call. The store then have to refer the matter to their public liability insurance company to investigate.
It would have helped if he'd gone to the GP, then the GP could provide a medical report - the insurance company may ask for one but he can cross that bridge when he comes to it.
Tell him he needs to write to the store now, and make sure that he says in the letter that he wants to claim damages for the injuries caused as a result of the store's negligence, and requesting that they pass the letter to their insurance company for further action.
I'm surprised the store rang and apologised that the floor was wet - that really won't be looked on well by the insurance company as it's an admission of liability. I can't promise he'll get compensation (I agree with factor's comments) but it's worth a try.