This is obv inconsiderate behaviour but I think it’s probably oblivious more than hostile. That’s not an excuse, and clearly shows a massive lack of self awareness, does she live alone? Its easy to slip into bad habits when it’s just yourself who has to deal with seeing toothpaste spit in the sink and bc it doesn’t bother her she’s not had the awareness that it would probably bother others in a communal area.
Neurodiverity gets bandied about a lot and pisses ppl right off as an excuse for negative behaviour, but there are a lot of ppl who have this, and they ppl who behave in ‘odd’ or non conventional ways are either entitled, lazy, narcissistic or ND to some degree. It’s not always obvious, especially if they are unaware they are and have not been diagnosed and appear to be NT, but for certain behaviours that other ppl can put down to being rude or ‘eccentric’.
I won’t go into the definition of autism or the origin of the name but a characteristic is to focus on oneself as opposed to other ppl, ie get the tooth brushing done, move on as it doesn’t bother them - its more a lack of social or self awareness, it’s not malicious, the negative reaction to their spit by others and their not bothering to clean it for the next person is just not thought about or considered (in SOME cases).
Also OCD and ADHD, specifically attention deficit, involves loads of thoughts skipping from one to the next. It’s quite likely that someone who has this would be brushing their teeth while thinking of something unrelated, finish, and already be focusing on their next task or idea or whatever and forget to finish the whole task of cleaning up after themselves. I’m not making excuses for lazy or inconsiderate behaviour, just trying to raise the possibility that it might not be as straightforward as ppl tend to assume.
As to how to deal with it - I agree, that’s difficult. Does she behave in a similar way in other aspects? Have you spoken to your mutual friend about how she behaved at work? It would be really awkward raising it so you can either try the talking about it in a non-confrontational, roundabout way - ie the fact cleaning, particularly bathrooms, PARTICULARLY THE SINK is an arse-ache, isn’t it?
Or not invite/agree to have her stay again
Or clean it up and say nothing and focus on the more positive aspects of your relationship/interactions with her.
You will have more of an idea if this is an arrogant person who feels cleaning up after herself is beneath her, or if it is a genuine oversight from a pleasant but often distracted or forgetful friend.