[maddermax]

He is convinced he has read another rule somewhere but we couldn't find it.. Any hard fast answer for this please?

Thanks in advance
Silence

The other rule he might have read was the one that is specifically for transport vehicles. If you destroy a transport vehicle, you are allowed to assault the unit that disembarks from it, even though it's not the unit that you shot at. This is specified at the end of the Transport Vehicle rules.

For all other times though, as the others have said, he's just stuck there.

Hope this helps