There is a passage from Cassius Dio regarding the surrender of Vercingetorix which describes that Caesar and Vercingetorix were once on friendly terms (40.41):

Now Vercingetorix might have escaped, for he had not been captured and was unwounded; but he hoped, since he had once been on friendly terms with Caesar, that he might obtain pardon from him. So he came to him without any announcement by herald, but appeared before him suddenly, as Caesar was seated on the tribunal, and threw some who were present into alarm; for he was very tall to begin with, and in his armour he made an extremely imposing figure. When quiet had been restored, he uttered not a word, but fell upon his knees, with hands clasped in an attitude of supplication. This inspired many with pity at remembrance of his former fortune and at the distressing state in which he now appeared. But Caesar reproached him in this very matter on which he most relied for his safety, and by setting over against his claim of former friendship his recent opposition, showed his offence to have been the more grievous. Therefore he did not pity him even at the time, but immediately confined him in bonds, and later, after sending him to his triumph, put him to death.

However, as far as I am aware this is the only source that provides such declaration which raises questions. The quote from Caesar regarding the surrender is as follows (Caesar, Gallic Wars. 7.89):

On the morrow Vercingetorix summoned a council, at which he stated that he had undertaken that campaign, not for his own occasions, but for the general liberty; and as they must yield to fortune he offered himself to them for whichever course they pleased — to give satisfaction to the Romans by his death, or to deliver him alive. Deputies were despatched to Caesar to treat of this matter. He ordered the arms to be delivered up, the chiefs to be brought out. He himself took his seat in the entrenchments in front of the camp: the leaders were brought out to him there. Vercingetorix was surrendered, arms were thrown down. Keeping back the Aedui and the Arverni, to see if through them he could recover their states, he distributed the rest of the prisoners, one apiece to each man throughout the army, by way of plunder.

Later writers have written their own version of such story, sometimes exaggerating (Plutarch, Caesar. 27.8-10; Florus, Epitome of Roman History. 1.45.26) but no mention of the relationship between the two.

Caesar informs us about the heritage of Vercingetorix (7.4):

There in like fashion Vercingetorix, son of Celtillus, an Arvernian youth of supreme influence (whose father had held the chieftainship of all Gaul and consequently, because he aimed at the kingship, had been put to death by his state), summoned his own dependents and easily fired their spirit.

Therefore, if the idea of Caesar and Veringetorix being on 'friendly terms' is true, it may have been during Caesar's time in Gaul prior to the rebellion in which a relationship between the two was struck up.