MWM, that is wonderful news.
But be careful - Autism Spectrum Disorders (ASD) kids don't GET better, but they can DO better. They ADAPT. And they can adapt brilliantly. Underneath, however, they still have Autism Spectrum Disorders (ASD). If you keep this in mind then you will be less likely to accidentally overload him.
About etiquette - get out some books, look stuff up online and maybe condense it down for him. Role-play some bits. But you shouldn't need to do too much because she DOES know him well, they are good friends. He needs to always have permission to be himself, but he CAN learn the things he may need to do as far as social rules are concerned. For example, he does need to know that he can't take the girl to the dance but then spend all evening ignoring her and talking to other friends. If they go together, they need to pretty much stay together. Not glued to each other, but he should keep her in mind (where she is, what she is doing) so they are neither of them left at a loose end for too long.
Don't feel bad that she asked him - these days that's OK.
difficult child 1's first girlfriend did all the chasing. She asked him to go out with her, they spent every bit of time they could, together. But almost always only at school. They were 'an item' for two and a half years.
What to watch for form here - if this becomes a romance, try to keep them going slowly. If it becomes too intense while they're still so young, she is likely to mature faster than him and 'outgrow' him, eventually wanting more from a relationship than he is ready to give. This is what happened to difficult child 1 with his first girlfriend. It was a mess for a while - he was suicidal, on antidepressants for two years. However, I talked him through it also, kept telling him that the break-up was not his fault, not her fault, just one of those things that had to happen because in their case, they had just grown apart. It happens. But having had the relationship last as long as it did - that was good, it showed that both of them were considerate, thoughtful, loyal and loving people.
easy child 2/difficult child 2 and her first boyfriend were an example of a relationship that was too intense that it burned out too fast. They were both too young, with too many issues that got in the way. It has taken two years but she is now no longer angry with him and he finally is over her enough to have another girlfriend. He is coming to difficult child 1's wedding so we will see him then and find out (hopefully) that he really IS over easy child 2/difficult child 2.
Part of this is normal typical teen romance stuff, but there are Autism Spectrum Disorders (ASD) overtones. One of the Autism Spectrum Disorders (ASD) aspects to romance - Autism Spectrum Disorders (ASD) kids tend to be extremely loyal. It is unlikely that he will ever be the one to break up with her (should they actually officially become boyfriend-girlfriend).
So encourage them to keep this on a friendship basis, but don't try too hard to stop things or it will confuse and upset them.
Tightrope-walking time!
It does generally work out, though. If he's being sought after for a relationship now, if he's been asked to be a date - then that looks positive for his future as someone's life partner.
difficult child 1's wedding is now less than 6 weeks away. girlfriend is a darling girl, she does have her own issues but she and difficult child 1 seem to complement each other well. He supports her when she needs it; she organises him and helps him learn to do things for himself. We've been working on this as well over the years, so at this stage she's slowly taking over some areas of responsibility. They will need some level of support with paperwork etc because I don't want the girl to have to feel she's got to do the lot; but she IS very independent.
MWM, with keeping him involved and doing the right thing socially, focus on practicality. Rather than say, "You'll break her heart," tell him that if he chose not to go then she would be disappointed. Being at a school dance with someone who is a good friend makes the occasion much more enjoyable. Having a friend who is also the opposite sex also makes life very interesting and a lot of fun. Being friends above all else is the best foundation for a long-term relationship, wherever things go in their lives. They may both grow up to each find someone else - but the friendship they have now, will teach them how to have a good relationship later on.
Mind you, easy child was 14, BF1 was 15, when they met and became "good friends". It quickly shifted into boyfriend-girlfriend basis and they are now engaged after having been living together for the past five years.
I am so happy for you all!
Marg