Q:
Setting the Stage Glitch
A:
Here is a thread with a couple of Dev responses: https://forum-en.gw2archive.eu/forum/support/bugs/Story-mission-changed/first#post4141401
Good luck.
Since he started the story instance, and you accepted “credit” for the specific way that the instance owner completed the mission (in this case, which is joining the Priory,) you joined the Priory. Nothing you can do about it now, afaik