"They met at a fashion event several years ago ... at least three years ago ... and became friends," a source tells PEOPLE. "They got together this summer after Jenna's [nine-year] relationship [with Mazeau] ended."
The source also emphasized that the Lyons-Crangi relationship "absolutely" did not break up the Lyons-Mazeau marriage.
"They separated two years ago, and Jenna went through several months of trying to work it out," says the source, who confirms that Lyons and Mazeau still live together in their Brooklyn brownstone for the sake of their young son, Beckett. "But two years later the problems just got worse."
A source close to Mazeau takes issue with that perspective, however.
"Like most marriages, they had their share of ups and downs over the years, but despite what's been reported in the press, they were not split before Jenna and Courtney began their relationship," the Mazeau source tells PEOPLE.
What was Mazeau's reaction to Lyons's news about Crangi?
"When Jenna told Vincent she was pursuing a romantic relationship with Courtney, it was quite a painful revelation, as Courtney and her partner and their children had been close friends of Jenna and Vincent's for over two years," the Mazeau source says. "They had spent many weekends, holidays and vacations together. Jenna and Vincent had even brought them all on a weeklong trip to the Caribbean earlier this year."
Feelings for One Another
D Dipasupil / FilmMagic
Just how is Mazeau taking all this? The source close to him tells PEOPLE: "Beckett has been his top priority for the past five years. His sincere hope is that they can all get through this divorce process as quickly and painlessly as possible."
Reporting by K.C. BAKER and ELIZABETH McNEIL