If its 30 years old, then you should definitely replace the furnace with a high-effeciency unit. It will save them money in fuel bills. If the unit is over 10 years old, it should be replaced. Even if you can fix the current unit, it may begin to nickel and dime you to death with repairs. However, if you don't like your in-laws you can fix it and wait for it to begin leaking carbon-monoxide.