If you friend can, I would also suggest getting a microchip flap for her own cats, something like a sureflap microchip cat flap. It will help to keep the aggressor cat out of the house. Keeping windows shut in the coming autumn and winter months will also help to avoid the cat gaining access to her house.
I do wonder, given the circumstances described of the aggressive cat's owner, whether the cat has in fact been neutered.
Whilst it's not a popular suggestion, there may also be an argument for trapping the offending cat, and seeing if there is a shelter who would be able to take it on as a rescue. They would be able to check to see whether it was neutered, chipped etc, and to find a good home elsewhere.
Otherwise, I think cat proofing her own garden may be the way to go, so at least her cats have somewhere safe of their own. The Feliway is a good idea, and hopefully will help to calm her own cats down.