She's right. I think you need to get a bike with bigger frame size or swap the frame you're using for something bigger and right for your size to reduce the amount of seatpost exposed.
Too much exposed seatpost is bad especially for big riders. Pro racers can afford to use undersized frames by several centimeters because they're light.
You can never destroy a seatpost or a seat tube by power. In fact, the more you make power or w/kg, the more you unload the saddle, unless something's terribly wrong with your pedaling technique. It's mostly the rider's weight, the severity of the road bumps, and how much seatpost is exposed.