Hello, guys.
A friend of mine recently stated that he could easyly charge on 1st turn with his Space Wolves.
The units in question are the Blood-Wolves-Bikesquadron (or something like that

).
The rules say that they always have to charge, if there is a foe within 6''.
Of course, as having bikes, they may use the Boosters to move up to 24''.
If they do so, the can't charge.
He argues that, although he cannot, he MUST charge, and so he does.
Well, although I'm pretty sure hat this is not possible, i'd like to request some hints to rules that clarify what happens if
something MUST, but CAN'T occure.
So far: Let yourself go!
Thank you!