With the electric tank LAST in series, only turning on the electric
tank during periods of peak use will not work, since the water after
being heated by the gas tank would cool and never be heated in the
electric last tank. espically is they didnt use hot water for a couple
days

if his electric bills spike he could add some valves, during normal use
water would enter electric tank first, and not be heated, going to gas
tank for heating. this way water wouldnt sit stagnant in the electric
tank. //// during peak use he would reverse this by changing valve
position to gas tank first and turning on electric tank. valves are
cheap, with 2 tanks of differing ages I would altering valving to
isolate one tank or the other, that way when one fails as it certinally
will one day he moves some valves and has hot water from the remaining
good tank till the bad one is replaced.