That was a point of contention on the show, IIRC. The prosecution argued that Adnan was jealous. Adnan claimed to be on OK terms with Lee, even giving her his new phone number shortly before her disappearance. IIRC, this was an angle of suspicion for the Serial producers, who found it strange that he didn't bother to call her after her disappearance.

(I'm on speaking terms with my exes. Even am still Facebook friends)