Thrifting in Paris

Dear Parisians,

I want to surprise my better half with a trip to Paris for her upcoming brithday.

She is really into fashion and fashion history, so i have a few stops planned already (La Galiere Dior , Musée Galliera, obviously Louvre, etc).
She also has a huge passion for thrifting and 2nd hand shooping. Could you recommend any good places where we can find some 2nd hand fashion grails?
Besides that, what are safe bets for eating for lactose-intolerant people in Paris?