Hello All,
My mom's Visa is expiring on October 2019. She is planning to visit on August 2019 to stay here for six months. When can we apply her visa in consulate office of Nepal? What documents are required?

she can apply six months before Visa expiry date. No document was required when my parents applied for renewal. Just fill the form and drop the passport.

If you're mom is going to visit in AUG no need to renew visa at the moment. She can technically stay for 6 months from the date of entry. US Visa validity only matters while entering, just make sure she doesn't stay past the date stamped by immigration officer at the airport. Also, make sure she has return ticket in hand just to be safe. Saying this from experience.
But like the other poster said if you want to renew, it can be done six month before expiry.