I have had chronic vertigo of varying degrees for almost two years with no clear diagnosis. BUT, I am now pregnant with our 3rd child and just before I got pregnant, I saw allergy specialists at USC who discovered I am very allergic to almost everything in the environment (no foods though thankfully). They were going to start me on weekly allergy shots but I have to wait now until after baby is born and I am done nursing because of the risk of needing epinephrine while they titrate up to my proper long term dosage.
Anyway, I clearly see a correlation with stress and I know the immune system attacks even more when you are stressed.
The doctors at USC really seem to think I have allergy related nerve damage in my ears, which is why the allergy shots hopefully will help.
I have been to many many doctors for the vertigo who tried to prescribe me so many different meds, it was absolutely ridiculous! And none helped one bit until I went the ENT's at House Ear Clinic at USC, and they put me on a special vitamin prescription of niacin (to dilate the blood vessels) and a special B complex (to help repair and prevent more nerve damage).
I highly recommend them. My doctor there is Dr. Derebery.
If you want more info, I'd be more than glad to help, but there are so many different possible causes for vertigo. I had to have MRI's, CAT scans, EKG's, Doppler tests, blood tests, EEG's, etc. to rule out things like brain tumors, seizures, heart defects, etc.
I will pray for your friend, vertigo is terrible.