I'd go ahead and sign up so your account is ready, then go from there.
If she contacts support, they should be able to just switch it over to you. That would be the way to go, since any other way could probably lead to downtime, due to not being able to add a domain name that's already in the system.
The owner (her) would have to be the one to initiate it, so she would probably just need your DH ID and the user name you want to run the domain under.
If she's also signing the domain name over to you, you can just do that through any registrar you choose. Nothing would have to be changed (other than the contact info) since it should already have the DN nameservers listed.
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