she has every right to be upset, it sounds like it was (as usual) kept 'hush-hush' to a newbie until a sale was made.
Basically spiders have this neurological problem, it is upto you to decide weather you think it's "fine" like everyone says.
We personally chose not to work with (ie reproduce) the spider gene because of this flaw. It has however become commonly accepted because people think spiders are pretty and (like they have all said) spiders seem to manage ok (eat, poo, breed etc)
To say it is not a big deal is wrong, I imagine it IS a big deal when you drop hundreds of dollars on your first ever ball morph only to find out about a well known neurological issue AFTER the purchase. You only say it's normal/accpetable because the breeders that are trying to sell these snakes say it is. Personally, just because they eat and shed (etc), it's no reason to class is as normal or acceptable for a ball python. Would you be happy with a normal that behaved like that?
The "spider wobble" is judged acceptable simply because it doesn't kill them, and because lots of breeders who invested $20-30 GRAND on their first spiders back in the late 90s wanted it to be a non issue and make their investments back.
If people bred mammals with such obvious defects they would be slated.
To just gloss it over to someone new to the hobby as "normal" and basically tell them to stop being upset is a little closed minded IMO.
I would also suggest that someone with an attitude like this has never seen a spider with a real bad case of wither the wobbles or 'corkscrewing'. The worst ones are truly heartbreaking.
What will become "normal" and "acceptable" next? Will the up and coming breeders of ball morphs be telling people it's ok to breed kinked caramels and caramel combos? That it's ok to breed a super cinnamon males with a duck billed face and a kink in it's spine like a golf club?
If the spider gene was discovered in the wild NOW with how the hobby is instead of being found right "back in the day" when the ball morph market was nuts would people still ignore this flaw? or would they say "we have so many other amazing looking morphs we'd prefer not to buy one that can't even tell which way is the correct way up". IMO if the spider gene was WC NOW, today, any responsible breeder would say "well this thing looks great, and it's genetic but if I can't fix this issue with new blood or outbreeding i'll leave it alone."