My in-laws forbade my fiance from having anything to do with the Mehndi and wont budge. Upon pressuring them a bit, their response was "Mehndi ki rasam is a hindu tradition. Aap yahi paisa jake Eidhi foundation ko dedein aur kisi ghareeb ki shaadi karadein."
You could always let them know that Mehndi isn't a Hindu rasm at all. There was no henna in India before the Muslims came...the Turks and Persians brought it and all the customs associated with it from the Middle East. And the way Muslims celebrate Mehndi ceremonies in Pakistan & India today is virtually identical to the way its still celebrated in Turkey.