I don't have the English BSB, but in the Spanish one it is clearly stated in page 56 (Close Combat Weapons) that the Cavarly lance is only for mounted miniatures. Therefore, the lance can not be used if the model is mounted, since its a Cavarly lance. Notice that there are different rules for lances (shorter versions), which can be used both on foot and mounted, but the cavarly lances (those that give +2S when charging) are only for mounted models.
If a model is dismounted (for example, if he is riding a monster that gets killed) then he discards the lance. For fluffy reasons, I guess it is too heavy to handle it on foot. It is legal to take it, but you can't use it (such as choosing a shield and a GW).