If she paid forward every bit of money flowing too her due to social media attention decided by the share of engagement derived from the photo, the artist would be Extremely happy and the case would be over.
These days attention is money, and this already gave her a ton of attention, not it’s giving her even more.
Emphasis on the important parts there, feel free to correct if I got one of them wrong.
Often they would not share the negatives or full resolution images. That way if you decide you want enlargements, more prints, whatever, you have no choice but to buy them from the original photographer.
BTW, in the above scenario it's highly advisable to get whatever pictures you want soon after the event. No telling how good your photographer's backup practices are, or whether the basement where they keep all the negatives will get flooded..
Copyright on commisioned works is pretty clear and I'm pretty sure you would have to explicitly sign over your rights in that case.
I think its more a matter of the photographer not knowing the law or knowing you are unlikely to take then to court regarding it.
The modern trend seems to be to follow the US law on this which is what you say but many signatories of the Berne convention in fact haven't changed their copyright law and commisioned works, specifically photographs, automatically belong to the commissioner in many jurisdictions.
Check your local laws.
I'm not going to go into much more but privacy protection laws likely apply here as well so that even if the photographer still owns the copyright they may not be able to make use of it since it contains your likeness,once again check your local laws.
I'm also reasonably sure it would be pretty hotly contested in an actual legal battle who owns the copyright to such works. There is most certianly prior cases regarding unfair license terms being removed or amended in contracts pertaining to copyright and this seems like one of those cases.
I standby my statement that the photographer is being shady and is assuming you won't take them to court over this.
One can obviously cherry pick a legal jurisdiction where the law is however they want it to appear.
Also Hackernews has audiences from all over the globe.
Regarding the initial point I made. Even if the commenter didn't own the copyright, they absolutely would have an implicit license to have their father paint a picture of the photograph and wouldn't have needed to see the photographers permission.
Also every legal system looks at international laws to determine how a law is to be interpreted. There's a ton that goes into anything so take it all with a grain of salt but I'd be very very confident that should the photographer even have tried to claim damages or gotten any recourse legally in this instance they wouldn't have any legal footing to stand on.