Grunger
I think your friend's golf club policy would be fine if they cut the last s from Seniors. There are then no apostrophe complications: Captain, Lady Captain, Senior Captain, Junior Captain.
I suppose for even greater consistency they should use Gentleman Captain, but historical sexism assumes the club captain is a man, so Captain it remains.
Although I think correct use of apostrophes is important, there are some grey areas. I don't think I mind its omission in a phrase like Parents Evening.