Dear Friend.
Deep Vein Thrombosis (DVT) can have many reasons, it can be Provoked or Unprovoked.
Check if he has any risk factors like,
hypertension,
diabetes, obesity, smoker, on any particular drugs, a long hault travel in bus or plane, bed ridden.
If not, he has unprovoked episode that requires detailed evaluation , your doctor must be evaluating him for that.
For treatment purpose, he must be receiving anti-coagulation. That will followed by long term anti-coagulation, duration depending on the cause of DVT, whether provoked or unprovoked.
Keep him admitted, prompt treatment is required as a clot can reach to lungs, leading to
pulmonary embolism, which can prove to be fatal.
Let us know,if you have any other query.
Wish your friend a speedy recovery.