What causes a person to pass out from having leg cramps?

Not enough info: Not enough specific ino to answer your question. Leg cramps do not typically cause one to pass out. You should see a doctor for evaluation.

A pulmonary embolus: This is by no means common, but if someone had a blood clot in the leg which then broke free and traveled to the lungs it could cause that person to pass out. This is just one possible answer to your question.