It depends a lot on what else is around the plot. If the house is reasonably near her last house, this is what I do:
Go to Google maps and put in postcode of current house. Switch view to Earth view. Look at current/old house's garden...by looking at shade pattern in old/current garden you will know approx time of day the photo was taken. Then scroll over to find new house and you will see whether the shade in that garden is much worse than current house, or not. You will also be able to guess how the shade would cross that garden according to what buildings/trees surround it, by looking at the shade patterns in surrounding houses and by just knowing that sun rises in east and sets in west.
Eg: in my last house, with small NE garden, entire patio was in shade by 12pm/1pm (depending on time of year). This is approx when Google Earth photo was taken. (Rest of garden quickly followed and entire garden in shade by 3pm). I've checked out all prospective new houses with this method - one had a NE garden which was entirely in sun at same time of day, and was actually better than some of the south facing gardens I was looking at, which had at least some shade at that time.
It also depends what time you like sun in the garden - I'm not bothered about the morning, I'd rather have it from lunchtime onwards on the patio area (although you can always have a seating area at the end of the garden if necessary) and my ideal would be as much at the end of the day as possible.
There are lots of nice plants that grow in shade, but as for food I do believe most things need at least some sun. However you can always consider the front garden for veggies, if there's space.
I would buy a NE garden again, if everything else was right about the house and the garden got at least a reasonable amount of sun (using my method above). It's not ideal, but I don't think it'll make the house hard to sell on...there's plenty of people who aren't garden people. I think it's more a case of south facing gardens attracting a small premium/being extra saleable, rather than north facing being unsaleable. A bit like having a garden room = extra saleable/small premium, but not having one doesn't mean your house won't sell.