Hello and welcome to the A/P!
I am so sorry to hear of your wife's experience, and the resulting anxiety it is causing her.
Fainting/syncope is a frightening occurrence, and a common fear of those with anxiety actually...
...so it is not so surprising to hear of your wife's reaction...and the fear that is lingering in regards to it.
However, you did all the right things in order to rule out any (other than benign) physical causes...so that is a great first step!
If she continues to have worries, you will want to encourage her to get help from her doctor for managing the anxiety.
We also have great self help available in our Resources
at the top of the main forum page here. Practicing the suggested exercises daily will help her to work through the concerns she is having. The 'Health Anxiety' links specifically would benefit her.
Encourage her also to talk with us. We understand, care and support is key.
Wishing you both the best!
"Courage is not the abscence of fear, it is feeling afraid and doing it anyway!"
"Worrying does not empty tomorrow of its troubles...it empties today of its strength."
Corrie Ten Boom
Panic Syndrome recovery due to CBT