@Feetupteashot and to everyone else who has expressed the sentiment quoted above
The whole point of @Maynot's Original Post is that the Grandmother has not shown any remorse, she has not @Feetupteashot said that she "feels bad about it"!
The OP herself has said that although she would still feel upset about the fact that the Grandmother reacted by lashing out, she would not be so worried about her son, who is on the Autistic Spectrum, spending time with his Grandmother if she (the Grandmother) had apologised for hitting him, and had shown that she wanted to understand more about his diagnosis, and how to deal with his reactions on the occasions when his reactions would not be socially acceptable, whatever the occassion happened to be.
However, instead of apologising, the Grandmother still refused to accept that she had reacted badly, and even said that, in effect, she didn't care about the parents wishes/rules, that she would treat her Grandson in whatever way she wanted to. Under those circumstances @Feetupteashot would you still "defo give a 2nd chance"?
If there are any parents here who honestly think it is acceptable for a Grandparent to go against the wishes of normal caring parents, of which the OP and her partner obviously are (I am of course not taking about neglectful parents, or even worse, the monsters we hear about in the news), then would you mind please giving your reasoning?
For Mumsnetters who have no personal experience of people on the spectrum, please believe me when I say that people who are on on the spectrum themselves, can have an awful lot of confusion about how the rest of the world seems to work, and what the "rules" are that they should apparently be following. I am not talking about the more important major laws that everyone who is going to be living in society has to learn to live by, I am talking about behavioural and societal rules that - even after many years as an adult - they may still struggle to understand.
For a child who is on the spectrum - and that spectrum has a very long and complex measurement line, that even experts in the field can have difficulty in knowing where to place any particular child - that child probably has a bewildering amount of information to take in, process, and hopefully eventually understand enough of what society expects, and wants from him or her, for them to be able to join that society and both be accepted as a productive member of it, but also as someone who "fits in" with their fellow human beings.
(Please accept my Disclaimer here - I am well aware that general society itself has far too many members who although they are NT, they behave in a way that is totally unacceptable, and not at all productive in helping society as a whole. Many of them don't deserve to be accepted by society at all. Unfortunately, some of them are so clever with their own presentation, and at manipulating others, that they are never discovered as being the ones that society should be rejecting, and not being allowed to live freely amongst us eg many politicians and others in powerful positions).
Now please think about the parent of a child who is on the spectrum. First of all, that parent (any parent, I am not talking specifically about the OP here) may also be on the spectrum themselves and not even realise it, or they might be completely NT -
I used to know a lady who was a child therapist who worked mainly with the medium of art, she once said to me (maybe outrageously, it was in a different era) that she believed that all men were somewhere on the spectrum -
but whatever label may or may not be relevant to them, that parent has to somehow, not just know how to bring up a NT child, who on their own have many complex needs and wants, but also to learn too pretty bloody quickly how to parent a child with even more complex needs, often without much, or enough, input from the 'experts' in their child's diagnosis. So please Mumsnetters, don't expect any parents of any children with additional needs, to be experts themselves in both any particular condition, and the way it may affect their particular child. With most neurological conditions the parents don't even find out about them at the time of their child's birth, it might be months or even years until they realise why their child doesn't seem to be developing within the norm that the "experts" on child development expect.
So basically, please just stop giving the OP such an awfully hard time on here, I should think that her life is exhausting and hard enough anyway!