Its been hard trying to find rules for this situation, but this is what Ive found.
Bottom of page 72 says that if a character joins a unit he 'becomes part of that unit'. I would say he follows the normal rules for the unit he's joined, so his arc of sight is reduced to 90 deg (as explained on page 18). If hes with skermishers, it's 360 degrees, and so on.
Bottom of page 73 says the character leaves the unit 'when he charges out of it'. So, until he moves, he's part of the unit.
It's not the most solid argument, but i think it's the best. If someone can find something else, then feel free to chime in.
Moglun, This situation would probably come up a little more if the character was right on the corner of a unit. If the flank was threatened by, say, fast cav, then could a character charge at them and mow them down? Id say no, as the unit cant see him, and the character is part of the unit.
Regards, Tim