You share a brilliant idea you had with a peer (in order to get some feedback on it). And later learn that they have decided to implement it themselves!
I'm already in the process of executing on it myself as well. My goal was to get feedback from people in the industry, not create competition.
Nowadays, ideas are worthless and execution is everything. Your situation wouldn't be any different if someone came up with the same idea in a different part of the world and you discovered it by stumbling on the site/product/ad. The fact that someone in your circles is using your idea sucks, but there isn't much you can do as they can always say they thought of it on their own.
The best thing you can do is build better, launch faster, and pivot smarter. Highly original ideas are very rare these days, so assuming you will have competition is just practical.
If you shared the name you had for an idea that is a good descriptive name, you would want to protect that - register a domain name, put a splash page up with "[name] coming soon" and file a trademark based on the name being used in business ($300-$400 with online services). And don't forget to register all the names on social networks with that name (check for those being available before finalizing the name).
To put things in perspective, I hear this happens to startups who pitch to investors - investors take the decks with proprietary info and share it with companies in their portfolio or even people in their networks who use it to launch products. These days most people don't bother signing or asking to sign NDAs because they are simply hard to enforce. It's a tough world.
Edit: Adding a couple articles to read on the topic:Why you shouldn’t keep your startup idea secret