In Chrétien’s version of the Knight in the Cart episode, Lancelot first fights Meliagrant the day after crossing the Sword Bridge and later in order to prove Guenevere’s innocence of sleeping with Kay; Bademagu gets both battles stopped in time to save his son. After the second fight, Meliagrant has Lancelot kidnaped and secretly imprisoned so that he will miss their appointment to fight a third time, at Arthur’s court.
Meliagrant puts an unnamed seneschal of his in charge of Lancelot. Eventually news of the tournament to be held at Noauz reaches the seneschal and his people, making Lancelot so moody that the seneschal’s wife, in her husband’s absence, lets the prisoner out on parole in order to attend it. She even lends him her husband’s new scarlet arms and fine horse. I cannot discover exactly where the seneschal goes during this time. Obviously she knows he should be gone for a while; but if he himself had gone to the tournament, why would he not have taken his own horse and arms, or at least recognized them on Lancelot?
Wherever he has been, he gets home a few days before Lancelot’s return. Much upset on learning what his wife has done, he reports it at once to Meliagrant, who trusts his prisoner’s word but, considerably annoyed that Lancelot has been out at all, proceeds to have a strong tower built on an island for his more secure imprisonment.
I would guess the Vulgate’s Roliax to be not Meliagrant’s seneschal, but whatever man or men brought Lancelot his food while he was in the solitary tower. Roliax’s wife might, however, have been derived from the seneschal’s wife, though in Chrétien’s version there seems to be no connection between her and the King Bademagu’s daughter, the latter finding Lancelot and enabling him to escape with no assistance but that of some careless worker who has left a pickaxe lying around outside the tower.