So long as he's using the proper rules for a LR and isn't trying to exploit it by having them in a different possition, in other words, its just a cosmetic thing, then I don't see why not.
Technically a Landraider pattern has to have the sponsons side mounted and has the weapons arcs of them being side mounted... but cosmetically, he can do whatever he likes with his army.
However, if he's trying to claim being top mounted they have a greater feild of veiw, or offer any other benefits not in teh official rules.. then yeah, its totally illegal.