Is it leaking oil, coolant, or both? I'm guessing it is weeping externally, as that is the common failure style of the 2000+ SOHC 2.5 engine.
If coolant: go to the Subaru dealer, buy a bottle or two of the coolant conditioner (rebranded Holt's RadWeld) and put it in.
If oil: switch to Valvoline Maxlife or Pennzoil High Mileage oil. Not likely to help with a gasket failure, but it may and they are good oils.
Also, if the Forester is anywhere near the 105k miles timing belt and assorted "while you're in there parts" (water pump, all tensioners and pulleys, crank and cam seals, reseal oil pump) I'd have it done then. The TB has to come off for a HG repair anyways.
Tips:
Have her contact Subaru of America if the car has been maintained well, she might get part of the HG repair paid for.
When the HG's are done have the heads checked and machined if needed.
Make sure that a Multi-Layer Steel HG is used in the repair, they don't tend to fail like the single layer original one does.
Check the fluids often and top up as needed. If it doesn't loose enough fluid to have overheating issues it can run a looooooooooooong time with an external HG leak.