My dad has a blood clot in lung, is this serious?

Depends. I would say a blood clot in the lung is serious but there are small clots and then there are large clots and depending where the clot goes determines how much damage is done and the severity of the pulmonary embolus. Figuring out the cause and where it coming from and getting on the right medication is the hard part.
Can be. Dr. Proffitt is right, it can be very serious. The good news is that with appropriate treatment, the outcome is generally very good.