If your friends furnace is a high efficiency furnace, it is supposed to make water when it runs and the water is piped through plastic hoses to a drain line.

You can tell if the furnace is a high efficiency furnace by looking at the vent pipe. If the vent pipe is made of metal it is a low or mid efficiency furnace. If it is plastic or PVC or ABS it is a high efficiency furnace. You may simply have a loose hose connection iside the furnace.