They can add charges but your son can dispute these if they are clearly unreasonable. It is best to put any disputes in writing as debt collectors have a habit of only keeping records of what they want.
Payments should be in accordance to whatever was agreed between your son and the collectors. If it was agreed to pay an amount every 4 weeks, then there is no reason for the debt collectors to raise this issue of whether the payment day is Friday, Monday or any other day, as long as the payments come at 4 week intervals. If it was agreed to pay every 4th Friday, then, by paying a day late, the agreement has been broken.
The escallation to the default notice just means that your son will get a red letter requesting payment of "arrears" otherwise a default will be added to his credit file.