I'm slightly surprised that TDS have requested they fill in a notification of a deposit dispute form as these are for when members of the scheme (usually agents) have been unable to get the landlord and the tenant to agree on how much of the deposit to return to whom. If the deposit is no longer registered with them I would not this would be the correct procedure (Though possibly they plan to have the entire deposit sent to themselves in order to ensure it can be returned to the tenant in full).
If they need any help filling the form in (it is rather long and complicated) I can give them advice!
Have they spoken to GDH about the return of their deposit when their tenancy ends? It would be a good idea to ask what will happen in their circumstances.
and thank you for the skill point!