Had a situation where a friend lost a game against an opponent with Lone Wolf. The opponent would Target Lock, reroll a blank with Lone Wolf and then if he got a focus or blank, would reroll it again with other dice under Target Lock.

I am 99.9% certain you can't reroll dice more than once, but want to confirm it here that you can't do that with things like Lone Wolf, Predator, Han's ability, or other similar upgrade.

Only cards that would let you use the target lock on a second roll should be something like Gunner or Luke crew, which is a seperate attack.

You can only reroll any single die once. So in this case, he was wrong.

Yes, you can only reroll a given die once. For future reference, the citation is: Rules Reference p 13, Modifying Dice.

It's perfectly legal to reroll one die with Lone Wolf, observe the result, and then reroll other dice with a target lock. There's very little advantage to doing so, however, and rerolling the same die more than once is always a no-no.