You clearly have a problem with this friend and have to decide if it's worth it to work this out with her. I certainly don't think that adults should be hostile and dismissive of kids or that it's okay to not like any child than your own.
However, I was just giving advice based on your OP which is different from your second. In your OP you wrote ...
"Her son is 6 months younger than mine and very quiet and placid. We were all round a friends house on Saturday with another kid a bit older. My boy kept pushing my friends little kid whenever he was playing near him, or sometimes he would just go up to him and push him for no apparent reason."
Now in your second post you say that her son is not smaller and was following and taking things from your son.
I can't know which way it actually happened so my only advice to you is that you can't control how others react and do things- only yourself. Perhaps you can take some time to think about how you want to deal with situations like this. With a child as young as your son, I would really recommend having a very short limit of maybe two times to correct before you leave somewhere. If he has to leave toys and fun he will get the message eventually that this is not appropriate behavior. I know that can be exhausting and more of a punishment to you but everyone I know who has tried this had it work out for them.
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