<human>: Can you condense the text into a few key points?

I'm sorry. My title is crap, forgive me.

A bit of background: 

My girlfriend is a very jealous person. If I so much as talk to a girl she immediately gets jealous and a little too protective. Because of this, I keep all of my friendships with girls mostly private from her. I would *never* cheat on her under any circumstance. I truly do love her and would never, in a million years, do anything to hurt her.

The situation:

I have a good female friend at work who is like an older sister to me. My girlfriend knows about this friend, and absolutely despises her, no matter how many times I try to explain that it is strictly a friendship and nothing more. Anyways, this friend offered to take me out to lunch in a few days (nothing fancy, probably just Panera or something) because my dad recently passed away and she just wants to hang out and chat for a bit. 

This would be all fine and good, except my girlfriend would not be happy with this. I would honestly have no problem hiding it from her like some of you may suggest, but I could never live with the guilt of going behind her back and hanging out with another girl. I haven't even tried to mention it to my girlfriend because I know she hates my coworker. I have never given her any reason to have trust issues towards me, but if I went out to lunch with another girl without her knowing she would probably never trust me again.
<bot>: got invited out to lunch with female coworker, girlfriend is the jealous type, would not like it if I went out to lunch with said coworker.