I and a fellow friend owned an account to buy packages from yamaha since we used Tyros 5. And so far we have been using Genos,
(Yamaha itself confirmed that each account will be activated. up to 2 info of two Genos, any two Tyros 5)
We split the cost of buying packages, and share each other's login information with the commitment that no one will arbitrarily change the login password. Anyone can use that account to buy the package you like and if there is no cost-sharing agreement, anyone who buys will pay for it.
You just need to make an agreement with the previous owner of Genos about re-sharing the login account information so that you can download the packages and install them on that Genos (Or add another Genos).
Your case is the acquisition of Genos from a previous owner who has pre-installed packages purchased from Yamaha. This is actually just a deal with him about sharing login account information. And of course I think it is reasonable to split the cost. If he later bought a new Genos (Or Genos 2, Genos 3 ...) then he still fully used the purchased packages with that account. Unless Yamaha creates a new file format for the new Genos and the packages are not compatible with the new format.