How did you learn to garden?
Did it involve making mistakes?
Killing plants?
Starting with projects that on the surfaces seem like the sensisble/ easier option like growing your own veg or growing annuals, but in reality aren't?
Appalingly badly thought out planting plans and combinations?
Overcrowded borders?
Or were you born with an encyclopeadic horticultural knowledge?
I grew up with a grandmother, who was nasty and judgy but a phemonenal gardener, who look down on Monty Don because he wasn't RHS trained, FFS.
I've taught myself through reading books, watching TV, experimenting in my own garden (and failing) and working alongside my chelsea gold medal winning friend.
That garden is your friends property and his project, and his alone. As will be OP garden if she purchases this property. She may learn to love it and become an avid gardener and custodian of what is already there, she may bulldoze it and turn it in to a 'boring' non descript grass and grey garden, or she may bulldoze it and come back to it later when the kids are older and create something truely magical, of her own imagination and of this time, not a museum piece!
You don't sound like a very nice friend.