I'd also like to know more about the DNA test; I'm assuming it's an Ancestry/23&Me/FTDNA/MyHeritage test given that it's the potential grandmother and not the potential father who tested.

It's possible that there could've been a testing error, particularly with trying to get a good sample from a small kid. But if grandmother has other matches who fit with known relatives, and kid has matches who fit known relatives (presumably kid's mom would've tested?), then that indicates the individal tests are valid.

And that said, the range for grandparent/grandchild match also covers auncle/nibling matches, so just because boyfriend's mother and the kid have a match in the right range doesn't necessarily mean boyfriend is the father. Maybe boyfriend has a brother; maybe his mother has a much younger brother who's the father of this kid; maybe the kid's mother is an unknown close relative of the boyfriend's mother -- a lot depends on whether this is a relatively small endogamous community or a community with lots of in- and out-migration.

(None of which takes away from the main point that LW should not be having a child with this man.)