The NB Black Diamond is definitely one of the better COSs that have been made. I cooked on one for over 15 years and it's a good little smoker to learn on with a few mods. If the rust is just exterior then you should jump at it for $50 or less, I think mine was $300 new back in the early 90s!
It looks like the gas ignition is gone and it looks as though it's never been under any cover at all so I'd probably ask for more pictures or if possible go and see it in person to determine how much work she'll need.
I know that mine is completely rusted through on the bottom of the firebox and I now use the smoke chamber as a charcoal grill.