Yep, the tip over point is a lot easier to reach with extra dead weight on the back. I can just imagine you going into a lean in the corner and having the 280lb pillion shift as they got scared mid corner.I'm a girl too! I ride a Suzuki GS550L... it's a smaller "naked bike" so not huge and not heavy.
That said, I recently took a friend for a ride.
I'm 5'2" and weigh around 130. The friend is 6" and (I found out later) weighs about 280 (!).
The biggest thing I noticed was that at the stops, I needed both feet on the ground, rather than one.
I always back it off by 30% when I ride with a passenger. Even a smaller passenger can unsettle a bike with ease if you commit to too much lean and corner speed. Hold something back until they're used to riding and enjoy the rush.