Just fixed my glitch - I killed the nearby Scorpions and then I was able to pass a speech check (after putting all my level up points into speech!) with Cass and she was then able to follow me - maybe it wasn't all that glitched after all.
The only trouble is, the Van Graffs want her dead, so as soon as I see them, with her as my Companion, they're going to kill her...
They are not

So you should be fine either way. It's a nice 2x quests done with a sort of positive outcome so it's worth it if you don't mind losing Cass.