Thanks for the clarification.
So, the answer to the OP's question of,
"Can he unbind it via a web browser, without giving me his login details?"
is, "Yes."
The previous owner can unbind it remotely from a web browser with his login details...
Does the previous owner doing the unbinding need physical access to the drone during the unbinding, or can they do it without physical access to the drone itself?
"Standard" RID is the Built in RID that comes with a drone, and Strap on or added RID Modules or boards do not count.
If the manufacturer includes RID as an operating function of the Drone it is unlawful to "switch it off".
Get yourself a Hanatora charger with a storage charge option.
After you use the Battery, Charge it to "storage" (60%) and if you don't use it, check it every month and charge it back up to storage (60%) if necessary.
On the day of flight charge...