Morels contain several volatile compounds which cause gastric distress. (Forgive me for not looking it up at the moment, but one of them is/was a compenent of rocket fuel, which teenage me loved.) They have to be thoroughly cooked to burn those off. Or else dried.

Specifically for soup - which is, arguably, their best use - most people won't saute morels long enough before adding liquid, so it's always best to use dried for that. Otherwise, standard, boring, dry-sautéed + butter until tender works great, and has never given me a hint of upset.

The instructor of your friend's mushroom course may have been giving maximally-cautious advice, rather than trying to communicate nuance to the general public. That's often a wise choice. :-)

PS. If you're at all interested in foraging mushrooms, buy a copy of All the Rain Promises and More, by David Aurora. (If you're elsewhere than North America, buy a local guide, too, but still get ARPM.) Aside from the mushroom content it's wonderfully entertaining.