Advise your friend to try to gather a group of family or friends who will give up a day or a weekend to starting the clearance.
In a garden, EVERYTHING GROWS so if you try to tackle it alone, you will never get anywhere, unless you practically work yourself to death, or never do anything else.
She has a house too, to clean, decorate, etc. etc. and presumably a life that contains other, more vital projects than reclaiming her private jungle.
If she can't find volunteers, then finding someone who loves gardening and who will be happy to earn a little pocket money is another idea I can recommend.