Sounds goofy to me? I don't believe even if it was just left loose that it would damage anything in the short time you rode the bike? Maybe, but I doubt it. I would suspect an out of spec or improperly preloaded headset. I find it a bit tricky to preload or tighten the headset bearings on a bike with a carbon fork because it has a plug that spreads as opposed to bike with an aluminum or steel steer tube that uses a star nut to help pull the stem down to preload the bearings. It's easier for me after doing it several times but still takes a bit more concentration to get it done. I wouldn't put much trust in the ability of your bike mechanic especially if he sold you a $350 fork for no reason? Any bike shop worth visiting would take the fork back off, admit they made an error, and come up with a fix. I would get a second opinion.

What model bike are you having trouble with?