The rules state "All reputation must have valid reasons provided for why reputation occurred" this means when a dispute is opened they need to have a valid reason for having performed the action. Not that they have to write a dissertation on the rep itself. So when the person is asked, why did you de-rep? (If there had not been a visible direct interaction) the member would have to state the valid reason for the rep. Such as IM, VM, email or PM exchange.
This is not the case here, so the rep stands.