I am rather elderly, 70, and I have a Black and Decker string trimmer, which I have used for about three years. It is lighter and much easier to use than a gas powered trimmer. I use it for trimming around brick-edged flower beds, out-buildings, light poles, etc. Probably gets used about 15 minutes at a time. The string lasts a long time between renewals, and is easy to refill from a big roll of string I got at Lowes.
I have had, and still have, a gas powered trimmer, but it just got too hard to keep it running. I have a 29 year old gas powered EDGER that I use to edge my almost 300 feet of curbing. I have a square corner lot, about 150 ft by 150 ft. It does a much sharper and easier job of curb edging than a string trimmer many use for this purpose.
I would definitely recommend a rechargeable string trimmer for this gentleman. I don't know exactly how long it trims on a charge; it does not run down in the 10 to 15 minutes I usually use it.