It looks like I have a different take on this. Especially since we are only hearing one side of it and that view is only from a third party.
It is normal for people with similar backgrounds to end up being attracted to each other. So while going to these meet ups to 'pick up' may not be your motive, it could very well be for others. That does not make it wrong. Whenever I have organized ex-jw groups I let the people attending decide the dynamics. If two people want to hook up, that is their business. I personally have not come to these things to 'hook up' but others have, who am I to judge.
As adults each person attending should all be capable of establishing their own appropriate boundaries. Basic conflict resolution dictates speaking with the person. If she has 'gently told him" (note for some this means different things) then it is time for her to ensure a clear message is sent. For example, you say he has called repeated times. Does she engage him in the conversation even politely listening or has she actually said, "I would prefer that you not call. Let's just stick to seeing each other at the meet ups with everyone else (or similar)."
Throwing out a few different perspectives on this: .. is it the age difference that bugs you? Your profile says you are 25 so are you 'interested' in this young gal or just naturally protective? Perhaps the 'middle aged guy' is still reacting like he did while in the Org where pursuing girls half your age is acceptable. Could i t be that part of the leaving process is learning better communication styles. Or perhaps this 'middle age guy' is in fact a perv and should not be welcome.
Lots of unknowns here and I did enough cutting off and judging while being an elder to actually support it on what I've read so far. In case anyone is so inclined to turn this around on me, yes I am middle-aged, never been to a meet-up in Minnesota and I have in fact been asked out by girls half my age and declined. Ten-twelve years difference is my max range and even that is pushing it imo.
A different slant but my 2 cents.