Some adults have just never been properly directed in this basic stuff as kids by their parents. I have a colleague at work who says that didn't realise she needed to do a lot of things until she went in the army. Also needed parenting classes as she just had never had that role model at home showing her the basics of caring for yourself and others. I also had a friend who was in boarding school from a young age. He went to uni and ponged when you got near him. He had lacked a parenting influence to make sure he kept clean, I suppose. Probably the house parents didn't get as physically close to them as a parent might so no-one told him?
Also, we tend to be quite hygienic and clean in this country, probably too much the other way, but there are some cultures who don't see the need to wash so often. If they move to a different country where people are washing daily and they are not used to that then the body smell differences will show. They may not realise it at first.
And before jumps in accusing me of saying that "all foreigners smell" and that I'm a racist , I would probably be the one sticking out in somewhere like Japan, one of the cleanest countries in the world. They'd probably think I was a minger!
Likewise, on the subject of bodies and what we do with them as a culture, I wouldn't get on with naked communal saunas in Sweden either, but it's totally normal to them as a culture.
Could/would you say if there may be a cultural difference? Or what else you know about the person's background eg family support, that might indicate they just don't realise that washing daily is necessary to stop you smelling?