Should be fine as others have already said. If she would like to hand in a CBP form from the flight on the CBP card-stock, she can use the english version and match up the fields with the Russian version. The fields will be in the same order as that is how a CBP officer reads a foreign language form. I give it a low probability the FA will have a Russian form. Also since your mother in law does not speak english and a Delta flight from France will not have a Russian or Ukrainian (guessing that is her native language) speaking FA, having her ask for a form might not work out.

You also might want to make sure she has your phone number so CBP can call you if they need translation.