Hi AG08,
I have the answer for your Grandmother. Get a pair of AR (Acoustic Research) Model AW721 900MHz Wireless Stereo Headphones. They will accept any line-level audio output (from a TV, portable CD player, whatever) and are supplied with a couple of adaptors to fit RCA jacks, headphone mini jacks and the like.
A wall-wart supplies the headphone transmitter/charger base, which recharges the NiMH batteries (included) for the headphones when not in use. The sound is quite good, certainly fine for your Grandmother, with adjustable volume at the headphone, tuning indicator light (for the 900 MHz signal) and other useful controls.
I think they sell for less than $100 but you will have to check. Full disclosure: these AR phones were given to me at an AR accessory press event. But I've tried them and they have very good performance for the money and are reasonably resistant to interference. Ideal for your application.
Regards,
