Mushrooms usually develop in cool, wet conditions. They generally disappear once dry weather returns. Your friend is right; mushrooms derive their energy from decaying organic matter. A rotten tree root could be the source. There’s no chemical way to get rid of mushrooms. If you dislike their appearance, keep breaking them off with a rake and hope that dry conditions develop soon. The mushrooms themselves won’t cause harm to your lawn, unless they grow densely enough to shade it.