My experiences with autism are quite vast and varied, due to many people close to me including ex DP, stepdaughter, business partner and someone who works for me having it, on top of working with adults with ND as a main part of my current job and previous jobs.
I can often spot it straight away, even on the phone, or across the room with people I haven't yet spoken to. Sensory issues are a BIG thing. NT people can struggle with the changes of temperature and environment of going from warm and dry to cold and wet of course, but to those with ASD it can be torturous. I am not saying that all people with ASD are 'smelly and unkempt' of course but I am saying that some are, and for those who are, it is because of issues like this largely. It can be made worse then by being bullied/ostracised at school for 'not fitting in' 'not being normal' etc etc and sensory issues amplify in unhappy children. Add in a not-great upbringing and it makes sense why ASD people would struggle with hygiene.
I wrote about this on another thread but recently I have met two people with ASD in a social setting. Both of them smelt unfortunately, one of just 'dirty body', likely it simply didn't occur to him that it is kind of a 'rule' that we wash and wear clean clothes before we go out for the day to a social event, and BO, the other one had clearly not washed his clothes properly despite having a smart appearance-perhaps a result of, as others have said, not quite picking up how to wash clothes properly and nobody had told him.
I think a lot of people don't understand how easy it is to become noseblind. I have a very sensitive nose. I went to a client of mine's house recently-bathroom smelt of urine-he hadn't noticed (also ASD).
My stepdaughter absolutely reeked at times and didn't realise it wasn't the 'normal' way to smell.
Unfortunately, affecting others with your smell and bad hygiene is non-negotiable.
My business partner comes over for nights/days out often and he arrives with the clothes on his back and a toothbrush and that's it. He is luckily one of those people who doesn't sweat a lot and I've never notice him smell.
Ex DP was the opposite, very fastidious about hygiene, some would say to the extreme-sensory issues were very much present but focused in other areas (certain textures and clothing and foods).
I think clear instructions like the ones outlined for a PP's parent's foster children, are your best bet here OP.