Trombophilia

Hi, my girlfriend is 23 years old and she had a blood clot running most of the length of her entire leg (as far as I understand) about two years ago. She was diagnosed with trombophilia and is now treated with Coumadin pills. She's probably going to have to take these pills for the rest of her life. She ha no family history of trombophilia she's aware of.

I've heard somewhere that in some cases trombophilia can be a fatal condition, a clot may break-off and travel via the arteries to vital organs, where it may lead to a heart attack or a stroke.

My question is, how often this happens in patients (if at all), what are the conditions that are necessary for such a thing to occur, and is my girlfriend living her life under constant threat of impending death (a bit dramatic, but I do want to know)?