Infinite Summer

what were the circumstance surrounding the breakup?
Is there any discussion of what happened, other than intimations that the relationship soured after Joelle's acid in the face incident?

There isn't really. This is the best gathering of facts I could come up with from the summary of the Molly Notkin section in Elegant Complexity:

Notkin says that the Auteur’s son’s excuse
for abandoning his relationship with Madame Psychosis was his
“accusing Madame Psychosis of being sexually enmeshed with . . .
the Auteur.” On pp. 227-8 Notkin “has no idea . . . whether Orin left
because [Jim and Joelle] were lovers or what.” According to n. 80,
Orin knows Jim and Joelle weren’t lovers (but cf. Jim’s role in “the
end [of the relationship]’s start” on p. 297 and Orin’s implied jealousy
when speaking to Hal at the top of p. 250). Near the end of the
section, Notkin attributes Orin’s departure to Joelle’s “traumatic deformity.”

_________________
"Because so that was let's see."

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