Seen/have seen

[Donny]

My friend insists "it was the best I seen" is correct. How can I best explain to him the use of seen/have seen/saw ?

[Tdol]

It's a simple rule - have + past participle (leaving passives aside)
Omitting the auziliary verb is fairly common among native speakers in many regions, but it would not be accepted formally, so they had better accept this before they write a job application.