Yes, all the 'follow the rules to the letter' crowd have shown up...turning what was a very basic practical question into a FT ethical quagmire.

I'm glad I don't work for any individual or Company that would have a problem with what the OP is suggesting they do...if one of my team came to me and presented me with this situation asking me my opinion or if it was ok, I'd probably offer a long eyeroll and ask 'is this was the most pressing business issue we need to discuss today? (rhetorical, clearly), now about that project you're working on with Client X, where are we at with that? Once the relevant business discussion concluded, I would likely say something like 'that's a nice thing you are doing for your father, have a good trip'...my team member will be relieved and probably think I'm a pretty good leader who really isn't in the business of sweating the small stuff or looking over everyone's shoulder all the time.

Further, if there was a fellow team member or Company employee on the same flight who saw and reported what the OP did, by coming to me, or say HR, I would again take the 'is this really the most important thing we need to deal with today' approach, 'doesn't Judy, Bobby or Suzy not have anything more important to worry about? and with emphasis suggest that 'they have been duly noted for their pettiness and desire to call out their fellow team member/employee, that isn't the kind of shop we run around here'

To the OP, this should be easy...both parties should board together at the lowest Zone on either boarding card, at the doorway of the plane
as one turns right and the other turns left, look to the J FA and say 'my father is going to take my seat and I'm going to take his in PY, can you please make sure he has a nice flight, he doesn't travel often so this is a real treat for him'...any normal human is going to look at you and say 'of course I will'....total nothing-burger.