Trifle I don't think this is a trivial matter.
OP another article which may be of use....
www.anxiety.org/is-your-child-a-hoarder
Please get her dad to read articles, especially to note...
"As hoarding behavior is considered a chronic disorder that is associated with significant impairment in adulthood, it is important to understand the nature of hoarding behavior in childhood. Better understanding of the disorder will facilitate early identification and treatment, thereby possibly curbing the trajectory of the disorder."
Your DP needs to get skilled up and get on board with helping your dd. He can't just have an attitude that it is OK.
Of course it is distressing when children are distressed. My son fell off his scooter today and I wanted to put some cream on his hands. He didn't want me to because it would sting (but I knew it would help).
I rubbed the cream in despite his tears and protests (not easy to do), I did it as quickly as I could. I talked to him about how it would help. I held his hand as we walked along (I always carry Germaline with me!) and we talked about how my warm hand was helping his hand to feel better.
So we swapped hands and I again talked up my healing abilities, even using the phrase HHW (human healing warmth). This all seemed to help him feel better about what was happening so by the time we got to the play date he was all better.
I know mental health issues are different to physical difficulties but I do feel sometimes we can learn from different things that work. My son does respond to positive talking about why things are good/things work well etc. So if some approaches work better with your dd in other areas, maybe try them in this.
I am also a great fan of distraction, which seems to work well with a lot of kids.
I do know with some mental health issues there are specific things to be aware of, which is why specialist help is needed, e.g. sometimes talking about things helps but for other issues not talking about them is better.
So please do ignore me if anything I say sounds unhelpful.