In the Curse of Strahd adventure, you learn from the tarokka card reading of an ally who is destined to help you against Strahd. These possible allies are enumerated on pages 15-17 under the Strahd's Enemy section.
This ally often doesn't know this and must be convinced or at least told about the tarokka card reading before they are willing to join you. This is mentioned either as part of their description on pages 15-17, or as part of their description in the area in which you first meet them. As a few examples, the book describes:
This card refers to Ismark Kolyanovich (see chapter 3, area E2). Ismark won't accompany the characters to Castle Ravenloft until he knows that his sister, Ireena Kolyana, is safe.
-- p. 16, Strahd's Enemy
This card refers to Clavin Belview (see chapter 8, area S7), the two-headed mongrelfolk. Clavin serves the Abbot out of fear and a perverse sense of loyalty. His job is to deliver food to the other mongrelfolk, whom he abhors. If the Abbot still lives, Clavin doesn't want to earn his master's ire by attempting to leave, and he refuses to accompany the characters. But if the Abbot dies, Clavin doesn't have any reason to remain in the abbey, so he's willing to come along if he is bribed with wine. Clavin provides no benefit to the party without his viol.
-- p. 17, Strahd's Enemy
If your card reading reveals that Sir Godfrey is Strahd's enemy and the characters persuasively ask him for help, he offers to join the fight against the vampire. Fate has unlocked his memory of the love that he and Vladimir once shared, and the power of that memory drives Godfrey to help fight Strahd and restore the order's honor. [...] Although Sir Godfrey is unaware of the tarokka card reading, he somehow senses that he must perform one final task before his spirit can rest with Vladimir's, and thus he agrees to help the characters face Strahd in Castle Ravenloft.
-- p. 139, Fortunes of Ravenloft
An so on.
However, there is one ally in particular for whom there doesn't seem to be any description of exactly how this character becomes your ally (there might be other allies who aren't given descriptions of how they become your ally, I didn't exhaustively search for a description of every single possible ally; I know there are some who will obviously help you, such as Ezmerelda, and therefore do not need a specific description about what it takes to convince them, but the ally I'm asking this question about isn't as obvious):
B. Tempter (Queen of Diamonds)
I hear a wedding bell, or perhaps a death knell. It calls thee to a mountainside abbey, wherein you will find a woman who is more than the sum of her parts.
This card refers to Vasilka the flesh golem (see chapter 8, area S13).
-- p. 17, Strahd's Enemy
Area S13 is on pp. 150-151, and the entirety of her description (ignoring the "boxed text") is this:
The woman in the tattered red gown is Vasilka, a flesh golem that has been exquisitely put together to serve as Strahd's bride. Characters within 5 feet of Vasilka can see the seams in her powdered skin where disparate body parts stolen from Krezkite graves have been carefully stitched together.
The Abbot is teaching Vasilka the finer points of etiquette. He also intends to teach her how to dance. Vasilka obeys his every command. She can't speak but lets loose an unholy scream if harmed. If driven berserk, she fights until the Abbot reasserts control or until she is destroyed. She has the supernatural strength of a typical flesh golem despite her smaller size.
-- p. 151, S13. Main Hall
Unless I'm overlooking something, there doesn't appear to be any guidance offered to the DM regarding how such a character would become the party's ally against Strahd. Sure, as a DM, I could come up with something, but I'd prefer to know about any possible guidance from the adventure, because currently I'm at a loss with how I would be able to narratively make sense of this.
Is there any guidance that I've overlooked with regards to how this character becomes the party's ally against Strahd?