I don't think you're gonna notice any performance issue while gaming with Window Defender. On my potato laptop newer games run poorly no matter what the antivirus is. I haven't used any resource hungry AVs though. I have WD set at High setting via Configure Defender and also manually set it to check for update every hour. So, it even updates while I play an online game like "Tom Clancy's Rainbow Six Siege" which won't be the case for her but still I face no lag/fps drop whatsoever. Besides, your gf's laptop is not potato like mine so she's gonna be fine with Windows Defender. Don't worry too much.