These kinds of comments should not be let past..........that is a HORRIABLE thing for him to say and not acceptable in the least, wither he means it or not! Little comments and things you can ignore but something like that is sexual harassment and should not be taXXXXX XXXXXghtly.
What I would suggest is you return to the head boss and let him know word for word what this person has said and that you want this sexual harassment to stop. Make sure you use the term sexual harassment as that will get his attention and get this taken care of.
I am sorry that you all are having such a hard time of it, it sounds like this fellow is working hard to try and get a reaction out of you. Which you want to avoid or you risk getting in trouble at work yourself.
When he says these things, I would look him in the eye and ask him if that is a threat.....then firmly let him know if he threatens your girlfriend with rape again you will press criminal charges. That should shut him up real quick! The fact is while a little bit of harassment may be over looked, threats of violence and or rape is not something minor in the criminal courts so he should consider that and stop.
For the time being try to remain calm, I know how upsetting something like this will be but the more calm you are the quicker this can be resolved without a big fuss.
Well that's good news, don't let them folks over there get you down! It sounds like you both care about each other and hopefully with time everything will work out. The last thing you want is to cause further stress so avoiding them and ignoring them should prove helpful. But make sure you talk to your boss about it again, and if he starts again then let him know you can press criminal charges if he does not stop.
If you need anything just let me know.