For his mobile data plan it wouldn't typically effect that.
Depends on his data plan though.
If he is using the hotspot built into the phone it will just stop working after the tethering allotment is used up. Which is different then the phones data allotment.
I would double check to see what data plan he is on.
There should be details that would match up to let you know.
I am fairly certain though there would not be any over use charge though.