How thoughtful you are to worry about your friend!
It's very difficult to remove all traces of nut protein - especially from upholstery. I would suggest using Clorox Wipes and elbow grease to remove it from the hard surfaces and vacuum the rest as well as you can. Generally, if you're driving while eating nuts, you're not touching the same surfaces a passenger would be, so that'll help, too.
If your friend is extremely sensitive, she most likely takes her own precautions as well since most public places will have people with nut residue touching things. For example: I wearing long sleeves, I don't touch my face, I never eat or drink in nut contaminated areas, etc.
Oh, one more thing I thought about... using fresh air rather than the recirc on the AC couldn't hurt.