But, given the overall cost of the car, their per mile cost is likely far, far more than any other car on the road. Since they are elderly, I assume they don’t drive a significant amount.
And here is the problem of EVs. To become profitable, they have to be driven a lot. But they cannot be driven a lot because they require a lengthy charging cycle. As result, they never cross the threshold of financial viability.
A Volt makes the problem worse because it is not a long range EV, and it is not an economy gas car. If those friends don't drive a lot (say, 10-20 miles per day) then they definitely will use mostly the EV mode. Let's ignore the cost of the hookup and of the energy. Then the car's use is free... but you paid premium of at least $20K as a lump sum, up front. This investment buys you right now 5,000 gallons of regular gasoline, and any econobox (say, with 30 mpg - let's be generous today) gives you 5,000*30 = 150,000 miles! This means that you prepay for all the gasoline that you will ever use in a regular car if you buy a Volt. Furthermore, nobody buys 5,000 gallons of gas and stores them at home. We pay as we go - the remaining part of $20K can be used for other purposes, invested or spent on something else.
And there is another little detail: if they drive the maximum EV range (40 miles) every single day, never use headlights or heater, never encounter wind and never climb hills, then they will need 150,000 / 40 = 3,750 days, or a bit more than 10 years to reach that point. I admit that after those 10 years and 150,000 miles a Volt becomes profitable... but only to a junkyard owner, as the car of this old age will be simply unsafe to drive. People who splurge on such luxuries will not want to drive a jalopy.
The executive summary is simple: if you buy a Volt, you will never see the gas savings for the life of your car, as the EV premium is higher than all the gas costs in the future. EVs should become much cheaper to compete with gas cars. Until then they are nice toys that people may buy only because it pleases them.