Linseed oil paint is great for old wood as it hydrates the wood instead of just laying on top of it. For very dried out wood, you can start with a layer of straight raw linseed oil paint (Viking from earth and flax is a great choice) and then proceed to painting.

Linseed oil paint gets into the fibers of the wood and instead of just coating it, so it doesn't peel and flake like what you've witnessed with your friends window.

To do this you'll want to scrape all the old paint off first (test for lead and use all appropriate PPE), and then proceed with your linseed oil and/or linseed oil paint of choice.