'Tart+heart' I did invite the destructive child (lets call him K) to my second son's party because I felt sorry for his mum and him and I projected what I used to feel with my first son when he was seldom invited to parties onto them. From what I can tell, K is just a pickle in school (mild literacy difficulties perhaps) and at home. Mum is sweet but weak and has little control over him and he is obviously a child who needs a supernanny type structure. I am pretty sure there is no specific SEN to explain his destructive behaviour as the school are very good at picking that up but obviously I could be wrong there.
Mum kindly invited Son 2 to K's party along with 10 other kids. Because of K's previous behaviour at school (had been naughty when in year 1 but I think he seemed to calm down in year 2) only 2 of us replied and went . My son had a lovely time and K seemed from what I could tell in the 5 mins I saw him) a happy little boy at a family type do in his own house so I returned the invite to my son's football party.
I have to say he was hard work - particularly in the last 45 mins . He was very loud, boisterous, wrecked the singing of Happy Birthday and this was more pissoffing because his mum was there and did absolutely nothing. K was also being racist (aged just 7!!!) to one of my son's friends. I found it really tricky as to how to manage the behaviour because his mum was sat there completely passive. (I am really strict with mine - but it's a tricky line to follow with other people's children). I have to say that my eldest son who has complex HFA would never have behaved in that way neither would any of the ASD kids that we know apart from one who sometimes gets a wee bit hitty when overexcited but whose mother would have intervened immediately and dealt with any situations.
BUT the upside is his mum felt included, her child had finally been invited to a party. She now greets me like a long lost friend in the playground (nobody used to talk to her ). Perhaps K developed his social skills. My son had a lovely time as did his friends who knew it was just K being K and he didn't do anything (apart from the racist bit which is clearly learned behaviour from somewhere and that I only found out about afterwards)that was untenable - he was just super loud and boisterous.
So good luck with your family party. I think you have been very thoughtful to provide a 'chill out' room. Unlike 'K' least your friend's son has a sound reason for any of his 'behaviour'. His parents are on a learning curve just as all parents are on what triggers their child off, what they can do, how long for, the 'blow up' signals. So yes you might get some plant pots overturned (perhaps move them????) and maybe for your own sanity move a few precious bits if you think you need to for practical purposes but it will be worth it for that fact that your friend will hopefully feel included knowing that her 'old life', before the diagnosis, is not completely disappeared into the ether.