50/50. Many tech firms are booming right now, and unemployment is low, so I imagine a lot of them are grabbing almost anyone coming in the door. "Project manager" sounds like the kind of skill that could be transferable both inside and outside tech, so if she wants to move into that industry (for the money or whatever) this could be a good move. And yes, if they hire her he'll likely get a referral bonus no matter what. 

However, tech firms have also become notorious in recent years for sexual harassment problems. With the lopsided male/female ratio, I imagine that her sister's having a much easier time finding willing partners than he is, so he could get pushy. I'd say you're more than justified in telling your girlfriend about your concerns and asking a lot of questions about the lunch, but you really should let her go ahead. This could be a genuine opportunity for her.