Burnt and drying leaves

I think at this point u can't do nothing more that hope she will recover. Make some holes under the pot, stop watering a week. But that rust could also be a mould?

I think your on the right track with root issues( only because of the leaf is bending down so much. Lockout will usually not have so much of a sad look. Stab a few holes with a pen in the soil to help get air to the roots and to dry it out faster. Could also be feeding, But not knowing the amounts your feeding its hard to confirm that. I would say let her dry out and not feed next watering. ***Double check your PH. Happy Growing