Here is an article about all breeds Aortic Rupture
The more specific information that applies to the Friesian is located on the Phryso article
There are currently no tests available to determine risk. Please see the article to learn about symptoms and diagnosis. As the name implies it's a very serious and fatal condition. ***Update, at Utrecht they have successfully used a small ultrasound scope passed through the esophagus near the heart to view the area of the heart that could potentially have the defect. The beauty of this is that it's non invasive and the horse doesn't have to be asleep!! I am not sure the availability of this technology in the US but calling around to major equine universities or clinics should get you some answers.
Also please note that there is a specific way to autopsy a Friesian to determine if they died of Aortic Ruptor so please contact Fenway Foundation.
Here is a Form that helps with some information from Fenway Foundation, if this link does not work please contact them directly FORM . They may have special instructions for you based on your horses symptoms so please work with them to help identify the source of the problem.
If you have had or know of a horse that died of Aortic rupture please share stories and information with us so that we can help other owners. Knowing their lineage will also help, but it is not yet determined that it's genetic. It could be fetal development or post birthing complications. Any information you have could help.
If links stop working please contact me and I can Email you a PDF
Research gate article A
Research gate article B