They had severely tanked in March after lockdown had started. Not a big issue, India would be able to handle it..
Such things are merely out of panic, it stabilizes immediately after a day or two. But that's not the case with a long, protracted war that may last a few weeks to a few years, or even many years. No one will want to keep money inside a country that has the potential to get nuked.
Not just foreigners, even rich Indians will want to get out if a major war happens, and that also includes their money. So all those who can run away will run away to safer countries.
War doesn't come easy. And in a country like India, only the poor and middle class will have to bear the brunt, which is why war is for the rich countries.