Trenton Station is in a dark high crime neighborhood. It is not in a safe area. I would avoid that station. Park in the adjoining Amtrak parking lot. You can check out the exact rates online.
Seriously? Enough with the fear mongering. To the OP, you & your friend will be perfectly safe at the amtrak station in Trenton. As a young woman, I certainly wouldn't wander around the neighborhood by myself in the evening flashing cash or with lots of bags, but it is fine in the actual station or walking to the station from the parking area. In fact I normally observe a few police officers in the station keeping an eye on things.
The other bonus of picking up someone in Trenton is saving on the bridge toll over to Philly from NJ. I think it might be up to $5 right now.