Can I take my father's phone away or delete his social apps?

My father has dementia (diagnosed). He seems perfectly normal most of the time, except for being slightly forgetful. However, there is one very concerning symptom. He's making up these elaborate stories. My mother died nearly 10 years ago and he was content on not looking for someone. Now that he has dementia he has been searching Facebook and every week he tells me he has a new "girlfriend". My siblings and I have noticed on multiple occasions that the conversations he has with these girls are one-sided. He messages them literally hundreds of times and they never reply. The conversations are sometimes dirty, but very detailed. One time he even said he was supposed to meet a girl, but couldn't find her house. They were then going to meet at a public place, but she never showed up. When we have tried to "call him out" on these conversations, he makes up excuses such as "it was just there, they deleted the conversation" or "they're using a fake name". This is really strange because his every day life is completely normal, except for these stories.

My siblings have been discussing deleting his Facebook, but there are mixed thoughts on that. I know of at least one conversation where a woman replied and threatened to call the cops on him, but she blocked him instead. Other times they don't see his messages at all because they're not friends on Facebook or they just outright block him. He then approaches one of us to say he "accidentally blocked someone" and we explain that, no, in fact they blocked him. I don't want him to end up in jail or robbed.

Advice?