@Chris752 Do you mean she has chosen the "pay less upfront" option and has paid half already and wants to pay the other half now? Or thinks she can make payments on some kind of schedule?
That isn't how it works. Airbnb will take the second payment on the date that they already would have informed her of. I don't think she can arrange to pay in any other way.
But regardless, this is not anything a host has control over. Tell her that payments are handled by Airbnb, not the host, and she needs to talk to customer service about that.