Over the weekend, U.K.'s The Sun posted photos of the exes' underage son Rocco allegedly smoking with friends in London while under his father's care.
"This is exactly what Madonna has been afraid of," a source tells PEOPLE. "Rocco still isn't back in school – and now this. He is just a rebellious teenager, but he needs guidance and direction from his father."
The teen had been on tour for a few months with Madonna, 57, but left to go live with his father, 47, in the U.K. in December because, as sources previously told PEOPLE, he prefers his father's laid-back parenting style.
However, when Rocco refused to return home to his mother for the holidays, a judge reportedly ordered him to return to New York for his parents to sort out his living situation. Despite the order, Ritchie has yet to return Rocco to the United States, and a source close to the singer alleges Rocco has missed some school. As a Ritchie source told PEOPLE last week, Madonna even visited her son in between tour legs to make a plea for him to come home.
As a Ritchie source told PEOPLE last week, "It's about a fundamental difference in parental philosophies," adding the filmmaker views his ex's "old-school" approach as "counter-productive" and "harming his self-esteem and confidence at such a crucial part of his life."
But a friend of the former couple – who finalized their divorce in 2008 – believes the custody squabble was caused in part by their contentious history.
"This is far beyond a situation about Rocco – it's about a situation between Madonna and Guy," the friend told PEOPLE last month. "They don't have a good relationship; it's more of a power fight between them and if he can one-up her. This is the best way he can do it."
Madonna and Ritchie are due in New York Supreme Court for a hearing March 2; reps are not commenting.