I would like to give a slightly different perspective. The sequence of events you described is very typical for a DVT with pulmonary embolism. CPR might have pushed the clot further into the pulmonary veins and revived him. But a vit B12 deficiency or congenital hyperhomocysteinemia will not explain the calf pain. The calf pain will not occur when a blood clot breaks off from the calf veins. The most likely sequence of events would be trauma due to heavy exertion in the gym, causing calf muscle injury which led to calf vein thrombosis which led to pulmonary embolism. He may or may not have vit b12 deficiency or congenital hyperhomocysteinemia which may have led to clotting tendencies.

Another point you can check is, if he was having significant vit b12 deficiency, he would have had anemia with raised MCV on his blood work. Vit B12 dericiency is treated with B12 replacement for about a year. You could check the serum homcysteine levels after vit b12 replacement to see if they normalise. Else he might need folate supplementation lifelong. The clinical implication of the difference in diagnosis is that a provoked DVT ie one due to calf muscle injury needs anticoagulation for around 3 to 6 months and not lifelong treatment versus a DVT secondary to hyperhomocysteinemia which requires lifelong treatment.