Clearing clot in leg?

Hello,

Someone I know seems to have developed a clot behind his right knee, as there has been pain in the area, numbness in the leg, some redness, etc.

From what I've read, the clots in the leg aren't too dangerous in themselves, but the real danger is if the clot or a part of the clot breaks off, travels up, and becomes lodged in the lungs, i.e., "pulmonary embolism", which can be fatal.

What can be done to clear away this clot (or any clot, for that matter)?

I've read about an "oak bark formentation" that can clear a clot in days, but I don't know anything about how to make it.

What do you suggest, in addition to cleaning up the diet and other long-term fixes?