The first thing that she does is play modernity cop. Every time John whines about how simply awful it is that Elly talks about how out of sorts being a homemaker makes her, makes a smug, self-serving comment about working women or defends Unfrozen Caveman General Practitioner Ted, Jean can relied on to give the narrow-minded blowhard a kick in his fat arse.
The second function relates to something that someone told howtheduck about Lynn. What the insider said was that she treated Rod's staff as if they were her servants. What this translated to in the strip is that Jean acted as Elly's representative at the office. This translated to her relaying messages, championing Elly's causes and telling John that Elly doesn't really mean it when she talks about wanting only practical Christmas gifts.
It's this 'looking out for Elly' thing that led her to carry the so-called idiot ball for a while in May 1991. As we know, John was desperately trying to make his distant relative Fiona more distant owing to her making a right pain in the ass of herself. Since the dumb bastard didn't do what he ordinarily did and blather on witlessly about his home life in front of his patients, she had no way of knowing that Fiona wasn't his mistress. Her need to look out for Elly and default distrust of John led her to gossip about how her ungrateful jackass boss was cheating on poor Elly with someone horrible looking. My guess is that the only way in which she'd learn the truth is when Fiona finally passes on; this would lead to the Pattersons also learning something that it would have been good to know at the time. More on that next time.