You will be aware that rule 36 is part of the Civil Procedure Rules which can be very complex and I think the best advice that can be given is for your friend to return and consult with the Solicitor acting on her behalf who will be aware of all the details of the offer and acceptance, it may be that the offer or acceptance was not clear or precise.
You are correct that payment should be made within 14 days and if it is not the Claimant can enter judgement for that sum, but take legal advice.