You've rather provided the answer in your post. If the connection is below the MCT, then the first airline will decline to check the bags through. Indeed, your friend will also probably have to accept that the airlines will not be liable for any misconnect either, so one hopes he has a flexible ticket.
The reason for this is not so much bloody-mindedness, but pure economics. If the bags are mishandled at LHR and do not make the VS flight, BA will be liable (as original accepting carrier) for all the costs of reuniting your friend and his bags.
So the moral of the story? Don't book connections under the MCT.