My wife (now 29 years old) has been on Coumadin for 9 years.
Two weeks after our first child was born, she began having very bad pain in her leg. When it kept getting worse, we went to the hospital and she was diagnosed with a DVT and put on heparin. She spent about a week in the hospital and was put on Coumadin. None of the tests that were done showed any blood abnormalities so after a couple months, they tried taking her off the coumadin. Shortly after that, she developed another blood clot in the same leg and was hospitalized again. At that point, they decided to keep her on coumadin indefinitely.
When she got pregnant with our second child, she was switched to daily Fragmin(low molecular weight heparin) injections for the course of the pregnancy since coumadin shouldn't be taken during pregnancy. Due to the stress from dealing with our newborn's life-threatening illness, my wife neglected her medication after the birth. She had another blood clot and a pulmonary embolism and was hospitalized this time for about 5 weeks. The doctors inserted a stent in the vein to keep any more pieces of clot from moving into her lungs. After she got out of the hospital, she was in a wheelchair for several months because she couldn't walk on her leg without pain. This is not the norm, but her clot had been very large and done a lot of damage to the leg.
I know that sounds bad, but keep in mind that the 2nd and 3rd time she had clots, she had not been keeping up with her medication. As long as she stayed on her medication, she did fine.
Since then, my wife has been very diligent about taking her medication and having her blood levels monitored on a regular basis and she hasn't had any more clots. We even had two more children without any clotting complications. She just switches to the Fragmin injections when she is pregnant.
Other than some occasional swelling in her leg when she walks too much (like spending the day at Disneyland) and the fact that she bruises easier than most people, she is doing fine now. But unless some new test comes along that can determine why she gets blood clots, she will be on Coumadin or some other medication indefinitely.
I wish the best for your mom. Now that she is being treated, I'm sure she will be fine. Just make sure that once she gets out of the hospital, she is very very dilligent about taking her coumadin and having her blood levels monitored on a regular basis.
EDIT: I believe smoking increases the chances of blood clots. So do things like birth control pills.