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The flashing green key on the dash shows there is a fault with the immobiliser system, until that light behaves normally there is nothing much that can be done to start the vehicle.
A fault I have come across a few times on these after a flat battery is the key has been forgotten, it requires HDS (Honda diagnostics system) and a security access code from Honda's MARIS system to be done ordinarily, although quite a few mobile locksmiths these days can access the vehicles systems and code keys so it might be worth enquiring locally.
As for the key keep being forgotten, from memory it was the micu (multiplex integrated control unit - fuses and relays smart box) that was forgetting the coding, replacing it would be one way, (the next person to connect to the vehicle with a diagnostics machine, which someone will have to, record the fault codes so if it happens again there is history for diagnostics), again this would need coding and most likely with Honda's own system this time as opposed to a auto keys person, but do ask the local providers.
The last consideration would be to find out why the battery keeps going flat to avoid it, hopefully.