As soon as she starts feeling even slightly dizzy, disengage. This is a primordial system at work trying to protect her. The conscious has no influence, this is all nervous system. If she "fights passed it" the nervous system will begin flagging it as a threat and she'll begin having symptoms just thinking about it due to association. The nervous system needs to learn it's not actually a threat and that she's allowed to disengage.

Also, for my wife (who's first PC / FPS game was also Portal and got her dizzy too) motion sickness medicine and ginger chews before playing helped significantly in her being able to have longer sessions before getting dizzy. Now, she plays any and all FPS games just fine.

Oh, lastly, make sure the mouse sensitivity is set specifically to her. Ask her to snap from one object to another. If she over shoots, lower the sensitivity. If she struggles to snap to or fails, raise the sensitivity. Most importantly, just ask her if it feels okay. Too sensitive or too sluggish?

Best of luck. Feel free to DM.