During the events of the The Witcher 3: Wild Hunt Lambert tells Geralt the story about his friendship with Aiden and his demise by the hands of another witcher of the Cat School, Jad Karadin. Geralt then agrees to help Lambert track down Karadin.
To the surprise of both Wolf School Witchers, upon finding Karadin in Novigrad, he provides another version of the story depicting Aiden in a much less favourable fashion.
Karadin claims that Aiden received payment for a contract yet did not fulfill it. His death was as a matter of fact a tragic accident since Aiden, according to Karadin, lost his temper and was thus shot by the panicking Vienne. Thus a decision has to be made whether Lambert should follow through with his revenge or whether Karadin should be spared despite the death of Aiden. It is up to Geralt to decide whether he backs up Lambert in his vengeance or if he wants to take no part in it.
Regardless of the outcome, Aiden is described by Lambert as "the best guy I have ever known".