@rjtx I’m sure these dizzy/fainting spells are troubling for both your wife and you.
I’m going to throw a bunch of questions your way in hopes of getting some answers for you.
There are a number of conversations in the Connect forum related to dizziness and fainting but it would be like grasping at straws to know where your wife’s symptoms fall in line with these.

She gets dizzy but doesn’t actually faint or pass out. At first thought it sounds like low blood pressure. Has her blood pressure been taken at the time of an event? It wouldn’t be a bad idea to purchase a home blood pressure cuff and take readings on a regular basis, same time of the day. Then if she has a recurrence you can check her BP at that time and see if it has dropped.

Does she have allergies which can give her vertigo?

You’ve mentioned she has low iron in her blood. Has she also been checked for thyroid issues?
Is there a pattern to when these symptoms occur? Is she sitting or lying down and it happens when she stands up?

Is she on any medications which can affect her blood pressure?

She’s had an MRI of her brain and blood tests, but has she been to a cardiologist for a full cardio work up to check for things like A-fib or HCM?

Have your wife keep a little diary of these occurrences such as what’s she’s doing when it happens. Time of the day, etc. Then a consultation with her doctor might yield some answers.

Some thoughts... Lori.