Have I overstepped here and what do I do

You’re really don’t seem to be understanding the advice you are being given. But you’ve been given some good ideas.

You keep drip feeding.

Bur essentially I agree with others that she may be making dubious decisions but it does not sound like she lacks capacity.
Her sexual choices are up to her. You can tell her you disapprove but she has the right to make her decision.
I have MH issues. I don’t leave the house. But I don’t lack capacity.

And you can say no thats gross, never mention that to me again. You can be honest with your friend, tell her you don't agree with what shes doing, and you feel bad for his family. But from what youve said she doesn't lack capacity for consent and outside of you making your feelings known to her its none of your business.

And you simply say ‘no thank you’. Job done. You can make your choices, same as she can.

You need to butt out. You’ve blurred the ‘vulnerable’ label into something different to suit your own moral compass.

Your friend has her faculties intact and you have no right interfering with her choices on her behalf.

Contact your local council safeguarding. They will determine whether there is a risk or if she has capacity. Tell them that police have declined to be involved.

I think plenty of people on the Internet tell celebrities that...

He's having a consensual sexual relationship with another adult. He's not flashing anyone. Morally, the pair of them may be dubious but this is so far out of your business it's not true.

You say nothing. You should not have posted on Facebook in the first place. It’s none of your business. How do you know he knows all about her condition? Why are you so involved? Even if he does know her situation in full, disabled people are allowed to participate in sexual encounters. It’s not for you to decide what she should or should not see or do. She could block him if she doesn’t want to receive the pictures and messages. There’s nothing wrong with two consenting adults doing what they are doing, as the police have already told you. Now maybe you will realise it’s none of your business.

This sounds very difficult but, if she is consenting to these messages, then there isn’t much you can do. You may find the photos and messages offensive but photos and videos of a sexual nature are legal between consenting adults. It sounds like
your friend has the capacity to consent and part of this is the right to make unwise decisions. It is not wise for her to be engaging with this man, but if she wants to she is an adult and that is her decision. Equally even if he has a wife it’s not illegal for him to send these messages and photos if consent is being given.

You would be better focusing your attention
on your friend and trying to help her to seek the support she needs with her mental health so that she doesn’t engage with men like this rather than trying to stop a man you don’t know. Your friend is obviously not in a good place and even if this man stopped contacting her there’s nothing to say she wouldn’t move on to another. Helping her get well would be a better source of your energy than going after this man.

Nothing you've said about her suggests she lacks capacity to make decisions about relationships? And yet you're contacting police and posting on Facebook because she's having a fling with someone she shouldn't be? Diabetes, depression and loneliness are all challenges in life but don't remove her agency. Butt out.

Ok now I'm lost. I thought it was tonprotext your vulnerable friend? If its just to shame him then go ahead, but you might lose your friendship over it. If I, as a diabetic with depression, found out my friend was messaging the daughter of someone I get my rocks off with and calling me vulnerable I'd be mortified.

He's not a weirdo because he engages in sexual practises you don't agree with

Your friend does sound like an adult who is able to consent to this, even if its bad for her. It's like blaming the off licence for selling to your alcoholic friend tbh

It's not really flashing if she consents...

Then tell her:

"I am very concerned that your father (if they are the same person - his name is (insert full name)), is sending sexually explicit videos/messages to a good friend of mine who is a vulnerable person.

I would appreciate if you would ask him to stop with immediate effect, as otherwise, we will be involving the police, due to her vulnerabilities, which he is aware of.

Thank you for your time."

Ah no it which case you have completely overstepped in my opinion. You cannot stop your friend from having sexual relationships on her behalf. It's not your business.

ita Nutty as fuck to suggest to his daughter that she should check his phone. It comes across that you are over involved in your friends business