Level difference in parties

If I'm level 43 what level does my friend need to be before she can play in a party with me and still get experience? she's level 21 at the moment. I was playing as a lower level class with her but she's had more time to play and has quickly overtaken me

iirc, you stop gaining experience if you have more than 10 levels more than the mob you're killing.

You can do dungeons and FATEs with your friend if you don't mind getting less exp.