What are some key road flat pedal bicycle adjustments that affect neck strain? Over time I grew to understand that different adjustments (saddle, handlebars, pedals, stem) affect different parts of the body. Often multiple at ones and often indirectly - by altering something else directly and then for example putting more strain on other parts of body indirectly because of that. My girlfriend recently started riding a road type bike (gravel) with flat pedals. Her neck hurts. I think my neck hurt when I became a roadie as well. But that went away after a few months. I'm not sure if I accidentally fixed my position then or if my neck muscles/whatever is there adapted.
I have tried changing her handlebar tilt upwards, and ever so slightly (1.5mm) moved the saddle to the front. I'm pretty sure there is nothing extreme about her position or the bike setting. The frame size is definitely good. I have also suggested that she moves her body around on the bike: stand from time to time, move the neck every few minutes, change arm position on the handlebars.
Does anyone have a good advice which parts of the bike can fiddled with to experiment and try to fix the neck pain?
If a rider is suffering neck pain, one of the first things to try is removing any visor from the helmet.  MTB helmets frequently come with some kind of sunshade/peak/bill horizontally over the eyes, and when on a road style bike, that means craning the neck up more to see underneat.  So unclip it.
Rider can also try to ride with a straighter back.  Its easy to hunch over so one's spine is curved.  That also puts strain on the neck.  At least try and straighten the back periodically while riding.
After that, it becomes a bike fit process, which is getting a bit much for an answer here.