Sure it is possible, however unlikely.
However, it is more likely that one (or both) of the parties involved was "recording/saving" the conversation and that somebody broke into their PC and got access to the file.
First thing to do is check their PC for virus/malware.
Second thing is if they have wireless networking to secure it. If they beleive it is already secured. non-broadcasting SSID and WPA code, then change the WPA code.