Hypo, only one guess from any one person is allowed between each of the riddler's posts.
"Sometime friend" makes me think of the GFs you have to fight to get them to join you. The second line throws me off many scents, though.
Cerberus was mythologically the gatekeeper of Hades. You could suppose hell lies beyond the gates in Cerberus' animation, maybe. And perhaps Diablos would be there too, lending more power to the line "at least one sometimes friend".
I dunno.It's sketchy, but is it hell/Hades/something like that?