Go with a standard GAS 75 or 100 gallon 75,000 BTU tank. If their
existing tank is 35,000 BTU this will double the capacity.

Tankless on low flow like one fixture running slowly are often below
the minimum trip level to turn on the burner or turn on the electric
resulting in cold water for these applications.

Another thing a electric tankless or most gas tankless no electric
means no hot water at all.