10,000 people? This would be the equivalent of getting a new patient every other day, all year, every year, for 54 years. Also, as 1 in 10,000 people are allergic to bee stings, I don't doubt that it's possible she's never had a reaction.
I'm not saying I don't believe in bee sting therapy, or even that it's a bad idea (I do it myself) but you need to be very careful when relaying medical advice from someone who apparently doesn't understand it. Just that she's been lucky doesn't necessarily make her right. And what makes her a "bee venom expert?" Pretty sure that's not a graduate course you can take yet.
Vitamin C is known to suppress cortisol production (by actual science). I'm not saying I don't think it's possible that vitamin C may aid in bee sting therapy, but so far, any information I can find (much like your own link) is only through word-of-mouth. No single scientific study has ever been done, and the reasons you give (ie: it INCREASES cortisol production) goes against what science has already proven.
But again, we don't know WHY bee venom reduces MS symptoms, but we know it does. (Scientific studies have been done on this.) So I'm not going to say vitamin C DOESN'T help either. But if it does, it's certainly not due to increased cortisol production caused by the Vitamin C.
I'm really not trying to be a jerk. I just have a medical research background, and information spread like this is how we get people believing that daddy-long-legs are the most poisonous spider in the world. (They're not. Even a little bit.)