One word for person you meet by chance

I am looking for a word for a person you meet specifically by chance. Ideally, a single word for someone you have never met before, but words for acquaintances would also be great.

An example sentence might be

They waved goodbye to their amusing [person-word-here] and continued on their walk through the park.


Solution 1:

“I waved goodbye to my amusing encounter”. This is a shift in the meaning of “encounter” but seems acceptable.