Ok, his words, well so far all we know from the OP is:
"I asked what that was about and he just said it's so weird that that's come from you " (my bold)
"I just grabbed the only thing I had which was a glass and let it go in there. I went to tip it down the sink and he started dry heaving and saying no why is your milk in there' ( my bold).
I don't see how either statement is automatically anything nasty, shaming, derogatory etc.
We have no idea how he said it, we have no idea how she said anything, we've no idea if this is exactly what was said or paraphrasing.
Theres plenty of stuff that we accept in theory as perfectly fine, normal, natural, but when we see it in person, it throws us - many bodily functions in fact, how many people have been totally grossed out by seeing a baby emerging the birth canal? I bet plenty and many of them women.
If I were retching over breastmilk and someone put it in a glass I associate with drinking out of, I probably would find that made matters worse and might well clumsily ask 'argh what did you put it in there for', between retches.
I might be panicking a lot if I had never considered that breastmilk in this context would make me retch (I don't know, feel fine typing about it but I've never seen it squirt out of someones boob or expressed into a glass directly from the boob so I really cannot say!) and so it came as a nasty shock!
OR yes he could be taking the piss and being a total wankpuffin, we simply don't know as the OP has not been back and probably won't be.