There shouldn't be any problems linking her account to your machine. Their exclusivity deal with Microsoft expires in November, so after that it'll be integrated into the PS3 firmware and you'll be able to toss the disc in the trash. I have my Xbox 360 and PS3 linked to my Netflix account and haven't had any problems.
My Dad has a PS3 -- he bought it mostly to have a Blu-Ray player and for the grandkids to play games on -- but never touched the machine himself until they added the Netflix streaming. Now he uses it all the time. He's pretty much working his way through every western they have. As soon as the Wii gets Netflix, he wants that disc so he can watch his movies in the bedroom while the kids are gaming.
Ah, technology.