It appears they have offered a settlement agreement, so they should also offer to pay his legal fees, or at least make a contribution towards them. That is because the settlement agreement will not be binding on him unless he has taken independent legal advice.
As there is a settlement agreement on the table, he should forget about the disciplinary procedure. That isn't relevant. What is going on is a protected conversation, which means neither side can refer to it if this ends up going to tribunal. The disciplinary process will only begin if he rejects the settlement agreement. They won't have to tell him what the accusation is at this stage, although it would be better if they did.
He will, as part of the settlement agreement, give up his right to make a claim against them for unfair dismissal, in return for which they will pay him an amount of money. This will be less than he would get if he made a successful claim for unfair dismissal, but it avoids the stress and the risk that he might lose.
The settlement agreement will also include other terms. It will, for example, probably require him not to tell anyone about it and include an agreed reference they will give to future employers.
He should neither accept nor reject the offer at this stage. His lawyer will probably be able to negotiate a better deal than the one his employers put on the table initially.