Justin couldn’t understand why his friend had refused to work in a company that offered him a huge salary. If it had been him in his friend’s place, he would definitely accepted such an offer.

The sentence is a third conditional about an unreal past ('If it had been him'), so the main clause needs 'would have' + past participle: 'he would have definitely accepted such an offer'. 'Have definitely accept' uses the base form, 'have definitely have accepted' doubles 'have', and 'definitely accepted' drops 'have' altogether.