I really doubt she wrapped baby up too warm on purpose both to give her "an excuse" to feed water and also to disguise a bloated tummy
- conspiracy much?? :o
Honestly what has almost definitely happened was -
She took your previous comment about water to mean "She doesn't need water at this moment in time", NOT "In 2019 it's strictly advised never to feed babies water". Because this may well seem completely unintuitive to somebody who was advised to feed babies water.
She dressed the baby warmly - as was advised when she had her kids, because the baby getting cold is dangerous.
She thought the baby was a bit warm (probably due to being overdressed, yes, but if that's how she felt it is appropriate to dress babies, she might have felt uncomfortable taking layers off) and she didn't want the baby be dehydrated, so she fed some water. In a bottle, because the baby is little, and probably in the typical bottle-feeding position which tends to invoke the sucking reflex, meaning the baby drank about 6oz.
If the baby was a bit bloated she either didn't really notice this, thought it was normal or just thought oh that's better, she's nice and full now. The fact the baby was crying was simply unrelated (which, well, it could have been...)
I doubt she is "too old to understand wi-fi" (FFS :o :o) but it probably wouldn't occur to her to google what babies are supposed to wear and whether babies are allowed to drink water. She just did what she did with her own babies, which is presumably what OP had trusted her to do? She brought them up at a time where you couldn't google guidelines obsessively every five minutes to check what you are doing is allowed, you just got on with it, maybe asking for advice from the health visitor or your own mum or other women you knew who had kids.
These things happen, it's just clashing assumptions, we assume that guidance has always been as it is when it hasn't, and they assume that nothing has changed, most of the time nothing bad happens. Chalk it up to experience, let her know more explicitly that you really want her to double check before she lets anything pass your baby's lips, yes even water, realise that she did it for kind reasons, not mad manipulative ones, and move on :)