Wow, that is a pretty high match. Second cousin, first cousin once removed, half first cousin, sibling of a grandparent... a few different possibilities. I hope the adoptee has figured out who their birth parents are already, that'll make the research easier. Otherwise, DDP has to figure out who the adoptee's parents are first. I hope the adoptee gets all the answers they may be searching for, and can help DDP to identify this UID.
I'm not sure this has happened before... I know for one of the other Does, who turned out to be Mary Silvani, an adoptee was mentioned as being one of her higher matches, but I don't think she had any this high? This adoptee is in this Doe's immediate extended family.
One other thing this makes me wonder... if this adoptee is willing to help in any way, would it be against the rules for the adoptee to show DDP their 23andme/Ancestry matches? Those databases are much much bigger than GEDmatch and probably have some strong matches. Would that violate some 23andme/Ancestry small print, or would it be fine because the adoptee is willing to show their results?