I have two friends with sons who are on the autism spectrum. One is 13, and does not like social situations. He has asbergers and dyspraxia (these are the diagnosis his mum has mentioned). He is perfectly happy when nobody is trying to put him into social situations. My other friend has a 10 year old boy. He sounds a lot more like your friends son.
The 10 year old is the same age as my oldest son. Because autism affects two boys my son is in regular contact with, and he is a mature 10 year old, I have spoken to him about autism. I could not do the same with my 6 year old. I posted about it here actually some time ago, asking for advice how to talk to him, and got lots of useful advice and links to show him. Because of this, he no longer gets upset by the behaviour of his two friends.
Having said that, the 10 year old, lets call him Peter, has been "difficult" since he was a baby, and he and my son does not get on very well. The diagnosis is new, and my friend is happy to know what is going on, and she is learning how to best handle his more challenging behaviours. His mum is not brushing over his behaviour, she is not pretending he is good as gold. She is not trying to force her son to play together with other children, but encourage doing something together. Like put on a movie and have pizza. Meet up at the skate park. There is always a purpose to the things we do, and the boys are not required to play together, or interact, other than in the framework of the activity we are doing. We often go out of towns for walks together. So it is me and her, her two sons and my two sons. Two pairs of boys of similar ages. We explore hedge rows, berries, ponds, rivers, have picnic, and in general enjoy ourselves together.
If my son and Peter are left alone to play together in the play room, well, this does not work. Peter will start wrestling my son, tickle him, snatch toys away, hog toys, refuse to share, call my son names, etc. Tease my son that he has something interesting that he is not showing my son, etc.
I think it works well because my friend is very in tune with her son, and together we manage to have a great time with all our boys because we select activities that we do and that all children can take part in.
It is a shame that your friend, OP, has given up on the Tennis and swimming, because these would be perfect situations where her son can be social with other children without the scope for much problems in interacting.
Why dont you suggest you get back to doing this? Or suggest country walks? Then you can chat while the boys run and explore?