You say it's not her first child. How old are her previous?
The old four month guideline was usually written in the past as 16 weeks (not 17 as now), so being a few days short of that would have been considered wholly unexceptional only a few years ago (2004?). And 16 weeks was an increase on an earlier 12 week guideline.
The harmful effects of early weaning are one of those population based measures that do not translate easily into specific additional risk for an individual child. This abstract from 2001 describes it nicely. But you will see that when it looked at actual age of weaning in UK, it was (quoting a 1995 survey) 2% of babies are given solids by 4 weeks of age, 13% by 8 weeks, 56% by 3 months and 91% by 4 months.
Those babies - who, horribly early though that weaning seems to us now, actually benefitted from later weaning than the babies a cohort before them - are now teenagers and I do not think they are that much healthier as a whole group than those say in their 20s (which us when I think 12 weeks was last the norm).
The duration of breastfeeding after the start of mixed feeding is interesting. This area is not studied much, and effects on later health (when interacting with when solids were started, and what types of solids at what ages) as compared to babies on non-human milk, are not I think known. (None of the population studied in the linked report combined BFing with solids).
The increase noted in rates of some poorer health outcomes seem (from this study) to be significant only after 4 months (not specified whether they mean 16 or 17 weeks). And of course was greater the younger the introduction.
I'd be interested BTW in links to further studies in the last decade or so that might cover those two areas which this study doesn't.
What your friend is doing flouts current guidelines but doesn't IMO warrant huge shock or concern, so I do think you're overreacting. (Unless you come back to add that she's telling other mothers to ignore the guidelines, in which I'd agree she is being foolhardy).