Okay during a recent game my opponent put his unit of B2's behind a wall apart from his unit leader then his whole unit attacked my troopers as his unit leader could see them since he was not behind the wall and therefore had line of sight.

Is this correctly done? Can a unit draw line of sight from its unit leader while the rest hide behind a wall and then shoot? And can I shoot them back, do they get heavy cover and can I still hit the ones in cover?

cover (and range) are measured from the leader, but each model must have los to at least a part of one model in the target unit to add their dice.

The Fifth Trooper did a great blog about this here:
https://thefifthtrooper.com/fundamentals-cohesion/

Range and cover is determined by the unit leader. LoS is determined by every mini in a unit. They need to see at least one mini in the enemy unit or they can't shoot/contribute to the attack dice pool. So no, you're friend was wrong.

Edit: Didn't see that Crewgar aleady answered. I'm confirming what he said.

Thanks I was pretty sure that was the case just wanted to make sure and couldn't find the specifics in the rules reference.

58 minutes ago, Flattened said:

Thanks I was pretty sure that was the case just wanted to make sure and couldn't find the specifics in the rules reference.

You can find the exact rule on page 16 of the current RR version, under Attack, on step 2:

"a. Determine Eligible Minis: Each mini in the attacker is eligible to contribute to the attack pool if that mini has line of sight to any mini in the defender."