Thanks for all responses. I was trying to gauge whether other families did this ... It's so nice to hear about @Skittlesandbeer family fun showers... I think this is great. It's the tiny shower cubicle, past things that made me feel uncomfortable - my gut feeling - but then again I don't want to be projecting my fears onto her !
From what DD said, if she was scared of the shower door being closed, he may have gone in there with her to show her it was ok, but why get fully naked? And why would it be a regular thing if it doesn't need to be. Just poor boundaries at the very least.
She can wash herself.. doesn't need her hair washed a lot - she usually has baths not showers anyway.
I have always joined in with her when she talks about willys and "Boys have willys ! " and stuff I will say "yeah ! " and just match her enthusiasm or curiosity about the subject without making it a bad thing. I think it's important to talk to children about consent whilst letting them have their own ideas and feelings about body parts and nudity... I've never shamed her for being naked herself or been shy to be naked around her, I just keep boundaries, to be safe and responsible, to let her learn that private parts are only for the person that owns them to touch ! Which I am pretty sure I have said this to her in joking in the past.
But I also grew up where my best friend's sister was sexually abused by her stepdad and I also was a shy little girl who was embarrassed by seeing my own Dad naked and still have like ughhh memories of it now as he was very liberal, and my parent's hippy friend's at festivals sometimes I saw, but no-one ever asked me how I felt about the grownups being naked and whether I would rather not see. Or gave me a choice...
My family wasn't very tactile or affectionate so YES I would have felt very strange at 5 years old to be having a shower with my Dad... it would have been completely out of the norm. But maybe he has made it normal for her... but is this right/ healthy past a certain age for a grown man on his own...whether it's his daughter or not, it doesn't teach boundaries around private parts or what is appropriate or not. If a man who wasn't her father showed her his penis maybe she would think it was ok, or normalise going past social norms of touching or closeness with an older male person !!!? If he hasn't had this conversation with her...
The other overly-enmeshed elements, lack of boundaries, lack of encouraging independence in their relationship worries me as a wider issue. But as other people have said there is no way of knowing if anything abusive has happened. But I certainly feel that given everything it's a bit inappropriate and he needs to be mindful ... I know I need to speak to him or someone..