Chris Brown -- Baby Mama's BF Arrested For Phony Checks

Chris Brown may have a leg up in his custody battle with his baby mama Nia Guzman ... her boyfriend was just arrested for forging checks.

Law enforcement tells TMZ King Ba was arrested for felony forgery, after he allegedly faked more than $5,000 in U.S. Treasury checks. We're told he was released on a $5,000 bond.

Ba told authorities he turned himself in after seeing his mug on Crime Stoppers.

As we reported, Nia has filed legal docs claiming Chris is not a suitable parent because he hangs with a bad element, but the sword may have been turned on her. For her part, sources close to Nia say she broke up with Ba.

Now all Chris has to do is worry about those drug tests.