Hi there!
Have you thought about wind!!! My DD started fussing over her bottles at around the same age and I tried absolutely everything. Apparently, when babies begin to feed, their digestive systems start working and their bowels start moving things along. If they are quite windy (as bottle-fed babies often are) then wind starts moving in the bowels and this can be quite uncomfortable. They then stop feeding and can even associate feeding with painful wind so refuse to feed at all - my DD went through a phase of this. I found infacol and Dr Browns bottles to be the most helpful.
Also (i don't think anyone else has mentioned it so I'll just drop this in for info), relux can get worse around this time and although her son may never be sick, he may get some acid moving up his oesophagus shortly after starting drinking which will make him uncomfortable enough to want to stop. Weaning can help some babies with this - and make some worse!
Hope she finds a solution soon. It's really stressful trying to get a baby to feed properly when they aren't so I really feel for her. My DD only really started drinking properly again at around 6 months and I'm sure I have many more grey hairs than I did!

I tried so hard not to push her but it's hard not to.
Good luck.
Cath
x