I know, I know... you said "cheaper the better" in your request. Tough.
Cheap headphones aren't what you need in this situation. Wireless, bone conducting headphones with a directional microphone and decent battery life is. If there's any way left to still tickle the old lady's ear nerves with vibration, and direct amplification isn't cutting it, these are it. If they work well where nothing else has, I suspect she'll go out of her way to take care of them.
AfterShokz Trekz Titanium.
Otherwise, the titanium headband, IP55 rating, and two year warranty will hopefully help fill in any remaining durability gaps she may be prone to.
If she already carries a smartphone, depending on the OS:
BioAid or
Mobile Ears for iOS (there may be better if you pay) or
Ear Agent for Android. The headphones are key... any trashed out cheap Android should fit the bill as the brains of the ad-hoc hearing aid setup for running an app that selectively boosts volume beyond EVERYTHING IS LOUDER INCLUDING BACKGROUND NOISE AND THAT WEIRD WHISTLING YOUR LEFT NOSTRIL DOES WHEN YOU TALK! (WHEEZE WHEEZE WHEEZE)