I understand what you are feeling and have been there myself. Unfortunately, I have sometimes, without intention, been the one who has 'disappeared'. Sometimes for days or months. Try to be understanding and nonjudgmental. I'm not condoning what this person has done. I'm just saying that sometimes 'Life Happens'. For example; my 99 year old grandmother lives with me for 27 years now. I try to live my life and do what I want. (I LOVE research and genealogy) Sometimes her health issues or mine come into play, and I can be taken away from my work and research without warning or choice. Emails pile up and people get touchy, mad or cut me off. Sometimes people intend to do well meaning things, but they are taking on more than they can handle. Sometimes, also in my case, They have had a medical issue that takes your short term memory. You get lost on issues and projects, with no idea that you were even on it, let alone what you were doing. :( Of course, it could be possible that they found out that they were just wrong, and they were not related and they are too embarrassed to say it or apologize. Hopefully not, but maybe they are hospitalized or in a nursing home or care facility; maybe incapacitated. God forbid, and I have had this happen, they passed away unexpectedly.
I would say, be patient, be kind, and hope that your paths cross again in a more positive way. Be thankful, you may have found a relative, if even for a moment, that you never knew of. You may have even found some info or leads that you would have never found.
If worse comes to worse, just let it go and know that your heart was in the right place. Your rewards will come to you. Let that other person reap the rewards of their actions; good if they meant well, and if not...well, they get what they deserve and you will not have to do anything. Just let go.
Best of Wishes,