If I had a signed listing agreement then I wouldn't be that worried about it, even though it would have been preferable for them to go through you before contacting the seller. I'd say it's a good indication that you will soon have multiple offers on your listing. With inventory being scarce, buyer's agents may be scrambling to find something to show their buyer before anyone else catches wind of it. However, in the long run that offer is still going to have to go through you. Counsel your seller, speak to the agent, take control. Good luck.