Thanks guys/gals who are responding! I'm pretty sure Jane realizes the seriousness of the problem with his temper tantrums, and the generally escalating violence that happens in domestic relationships with guys like John. If she's not, I've attempted to massage it in.
I'm just glad that everyone seems to agree that this guy is total bad news, and he's just going to get worse.
As far as recommending me as another guy, thanks for the vote of confidence,

but she doesn't really need another guy. What she needs right now is to leave the one she's with so she can be safe, and if trying to figure out what guy is right for her is messing with her head, all it's going to do is delay the most important thing, getting out of there! I didn't intend this thread to be a matchmaking machine.
