if she wants something light my 85 year old grandma would use this ryobi.. I had to buy her more batteries she went at it for hour(s).. at 85.
www.directtoolsoutlet.com
Ryobi batteries are the cheapest around if they need any other tools.
I love the 3gal shop vac for small messes.
2pack of 4ah high power batteries 99 with charger at home depot.(sale)
usual price of 2-4ah reg batteries is also 99$ at home depot.
Otherwise the greenworks 80volt line.. the costco one has 8yr battery warranty.
I've been satisfied with my dewalt 20v trimmer except the bump advance broke after 2 years (new part costs about $30)
but would only recommend it to someone that already has the batteries.. otherwise its mediocre.
I also find the dewalt easier to maneuver vs my gas craftsman and I get nice even trims.
it has 2 speeds low is enough for grass.. and high is for overgrown areas.. it also has a variable trigger(throttle)
1.5ah batteries are not big enough IMO need 3ah or bigger. I use some old 5ah batteries and if its fully charged I usually have around half left.
Unless I didnt trim for a long time I only use the gas trimmer for edging now. (edging attachment)
Any of the electrics will have 1/10 the noise and even less vibration so much nicer to use.