The fact that you previously listed this property for the seller and showed this buyer the property does not permit you to contact the owner directly.
Under the Real Estate License Act, a licensee may deliver an offer directly to an owner who is exclusively represented by another broker only if the owner’s broker consents to the delivery and a copy of the offer is sent to the owner’s broker. Any negotiations of the offer must still be conducted through the owner’s broker.
In addition, Standard of Practice 16-13 in the REALTOR® Code of Ethics provides that all dealings concerning property exclusively listed shall be carried on with the client's broker and not with the client, except with consent of the client's broker or where such dealings are initiated by the client.
Since the owner in this situation is now represented by a new broker, you should deal with the new broker concerning any offer a buyer might want to present.