I so agree with this.
I think most of the respondents on this thread are a bunch of cunts, to put it succinctly. Telling this mother off for disability discrimination while....
Well, in my mind, this poor mother is disabled herself. She has never been taught about disability discrimination, and so has a real handicap (yes, unaccepable word) in dealing with the world. She written a text which clearly, to me, says 'I was scared of this visible difference'. FFS she is asking for help. Yes, not in a way which we find acceptable, but the request is definitely there.
The information about this particular child's differences may not be yours to share, but the resources highlighted on this thread certainly are. I would respond something like this:
Dear mum
I do have to confess that I was a bit shocked by your text, at first. But then I realise that we all have areas of life where we have missed out on some vital information. You've been great at asking for help in dealing with distress at a visible difference - I suspect your own distress was the issue, as all the children at the party seemed to be quite happy. Children are quick to relate to the humanity in others, and may ask questions about differences, but their open hearts mean that they can adapt to a very wide range of circumstances.
Humans do of course have it programmed into them to avoid those who are 'different'. We needed this when we were at much earlier times in evolution, as physical difference can be related to infectious disease. We have moved on, though! What surprised me about your text was that our generation has been brought up much better - to appreciate that an accepting society is very important, and as mums we have a responsibility to pass on good messages to our own children.
So we, and our children, accept visible differences in others. We work to help people feel accepted and valued, not to exclude them or to 'give a heads up' if they were going to attend an event as if they were monsters (this was the phrase that quite upset me, but I do appreciate we all have things we don't know). I was touched that you realise that you haven't learnt more about difference and disability as you grew up, and yes I have plenty of suggestions for things that you and your children might both enjoy.
[list to follow, Wonder, all the resources helpful MNers above have suggested]
This is just a working draft. I've tried to be honest about my own shock, but to acknowledge areas of ignorance in others. And we all have these, and we might be shocked when we discover them.