Hi.. Welcome to healthcaremagic community forum..
You haven't mentioned your dad's age and if he is on any anti arrhythmic pills.
can lead to ventricular arrhythmia
and pulmonary embolism
. Both of these are life threatening complication.
The chances embolism are low as your dad is on anticougulants(blood thinners) with INR levels of 3.5, however, the risk of arrhythmia remains. Unless he is on medications to control the fibrillation, it isn't safe to go on a dangerous trip like hunting. The risk involved is very high.