Oh man, that's a classic shop tale. My cousin tried the same trick on his truck's hood after a bad storm. Ended up with a weird shiny spot and a dent that looked worse, like it got a sunburn. Those DIY methods sound good until you're staring at a messed up paint job. In my experience, it's better to just call a paintless dent repair guy from the start.
Listen to your buddy from the shop and you'll end up with a car that looks like it has a weird plastic surgery scar. The heat gun trick is a great way to cook your clear coat, which is exactly what @the_piper is talking about with that shiny spot. You basically melt the finish to make the metal move, and then you're stuck with a permanent shiny reminder of your mistake right next to the dent. It takes a real pro with the right tools to massage a dent out without wrecking the paint. That shiny ghost dent is way more obvious than the original hail ding was.