I've seen a fair bit of passive-aggressive behaviour amongst people in my community. Agree it is all about infantile methods of regaining attention. At some level also he feels or has felt powerless and has learned that behaviour as a coping mechanism. His own mother perhaps? It's almost a 'you aren't looking after me as well as you should', isn't it.
It's not really about the minor things at all - they just present opportunities.
Agree that the counselling idea won't work unless he acknowledges first that there is a problem and he owns it. Maybe he is so habituated to it that he can't see what he's doing at all.
However, perhaps he is unconsciously exploiting the fact that they are in your house to make her feel worse than ever - it's a lever.
Have you tried Harka's suggestion about confronting him yourself and pointing out his behaviour to him? This is more likely to work than your daughter doing it.
Beyond that I would say your daughter needs to stop rewarding his bad behaviour by paying attention to him, or appeasing him - but to stay centred in her own happy self and go off and do her own thing. This is not easy and takes some determined effort.
I suppose the flip side is also to reward him when he manages to deal with 'stuff' in a balanced, adult manner.
Ultimately and inevitably, if he continues to behave like a child your daughter will lose respect for him terminally and will not want to stay with him (whether she realises that yet herself or not). If I were her mother, that's what I'd tell him. I'd tell him my daughter wants a man for a partner, not a child. I'd shame him with some raw home truths, but I'd do it in private.
First, a physical checkup is needed by a physician who must know the symptoms and feelings of the person. They may actually be depression, a medical condition that can be treated. Or, as others have answered, other medical conditions that are expressed throughout of control emotions. If so and if treated, the passive aggressiveness will gradually disappear.
A passive aggressive personality or stage of development needs to work out their negative feelings and build up their sense of self, with a Counselor or professional with well tuned counseling skills...never giving advice, I mean.
You will be amazed how positive that will be. No one can make an adult go to a counselor...perhaps a person who has benefited as she needs to, can share positively and inspire her to see a counselor. A parent, however, can take their child to a counselor. If the child is 13 years of age or more, the parent should visit with the counselor first, then allow the child to go into the session alone. The counselor may give you information and may not; but you will see the benefit you want.
Are you sure he is not suffering from manic depression. This can cause violent mood swings and they can be controlled by mood stabilising medications. Would he agree to at least discuss the possibilities with a doctor. Are his mood swings irrational.ie if he is having an argument about something then a spike in temper is to be expected. But if he is in good form one moment then starts to get angry for no apparent reason, this could be an indicator for the manic condition.
Is there a counselling service for non-married couples?
If we were talking about a man I would think he were a psychological bully, or he wants to break it off, but not be the one to "do" it, or he feels bad about something and guilt is making him act like a git.
Given the opportunity, I would have had a quiet word, pointing out that the behaviour affects everyone in the house and good manners at the least dictate that you try to be a pleasant visitor.