Scubadoc's information nicely summarizes the use of Coumadin and diving. The big question, as he said, is what are the risks from the underlying disease that created the clot?
Coumadin is not an automatic contraindication, and having had a blood clot in the veins of the leg is also not an automatic contraindication. It would take a detailed review of her medical history and an examination to know whether she could dive.
One note for those that may dive when taking coumadin;
The degree of thinning is very dependent on diet and is also affected by a number of other medicines.
If someone normally ate very few vegetables containing Vitamin K, but made a big change in their diet when on vacation, the INR (degree of thinning) can change rapidly.
Keep diet as consistent as possible and check your INR regularly.
If you are contemplating a two week or three week dive vacation (color me green with envy) It would be wise to make arrangements for protime testing during the vacation.
If you're going somewhere really remote, there are new devices (Coaguchek and others) that allow rapid INR assessment without super-elaborate lab equipment, and can do it on a fingerstick blood sample.
Here's to bloodless diving;
John Reinertson