More than likely he was the one who got obsessed with her and pushed for it to be more serious, if she's a dead ringer for his wife then he could have been massively drawn to her for that reason, if she had dodgy intentions from the get go then how much of a coincidence would it be for her to happen to set her eyes on a widower where she had such a strong resemblance to his ex wife.
If she's kissing him in front of the children then presumably he's kissing her back in front of his children. Why is that her fault? You said "what kind of woman snogs a man in front of his bereaved children? but don't say anything about him doing it?
You say she seems to ignore the fact that there are photos all over the house of his late wife and her belongings are everywhere. I bet you would also have an issue if she had demanded that all that stuff was thrown out or removed from the house?
You say they're even sleeping in the same bed where the wife died? I don't know what the norm is there, but do people generally buy new furniture etc. when partners die?
He pays for everything but sometimes that happens in relationships if one person has more money, it doesn't mean that they're being taken advantage of, maybe they really want to do it, maybe your friend wishes he had spent more money on his wife when she was alive, bringing her away for weekends or exotic holidays and now tries to enjoy his life and partner more?
When you say she has taken control of his working life what is she doing? Could she just be supporting and encouraging him?
Her friends might be equally as worried about her, it sounds like an intense relationship, they might worry that she's with a new guy who got serious about her very fast, appeared to pick her because she looks like his late wife, spoils her with trips away to keep her sucked in, doesn't have much regard for his kids feelings as he snogs her in front of them, has his late wifes possessions everywhere, even the same bed....and yet his new girlfriend doesn't listen to the warnings!