Yichud after the fact

[Akiva Fleischmann]

Hi everyone! Hope people still read these...  

Someone approached me and asked the following question: He was staying at a couple's house for the weekend and the husband had to leave town suddenly on Saturday night, leaving his wife and this guest at home alone for the one last night of this guest's stay.

What is his obligation here? בעלה בעיר wouldn't apply, so it seems to be clear-cut yichud, but is there any significance given to the "awkward/embarrassing" factor of bringing it up and leaving for a different house?

Also, if he had had his son or daughter with him, could he have had his child sleep in the room with him? I know it's not exactly the same as having a child sleep between you in a חופת נידה case, but is it similar enough that it could work בדיעבד?

Kol Tuv!