My brother-in-law lost his wife to cancer last year (his wife was my husband's sister). He's in his 40s so naturally we expected him to find someone special, eventually, after recovering from his profound loss

My brother-in-law lost his wife to cancer last year (his wife was my husband's sister). He's in his 40s so naturally we expected him to find someone special, eventually, after recovering from his profound loss. Just weeks after her death, however, his wife's friend began inviting him out ... often. She made frequent offers to have dinner with him, drinks, etc. We all assumed she was a dear friend whose only concern was to lift his spirits. We recently found out, however, that he actually didn't know her very well at all prior to his wife's death.
My brother-in-law and his wife were both executives and because their professional lives were so demanding, they never had children. They accumulated considerable wealth in the 20 years that they were married. This woman is aware of my brother-in-law's financial situation.

For the last year, we have found it difficult to stay in contact with my brother-in-law. The new woman monopolized all of his time, preventing him essentially from seeing anyone but her. He happens to be a favorite uncle of my children, and when we suggested that we visit for a long weekend, he was thrilled. He seemed to really enjoy maintaining the connection with us and he was excited for us to meet the new woman who seemingly rescued him from his grief. Needless to say, we were not impressed.
It was there we learned that she had just separated from her spouse shortly after my brother-in-law lost his wife. We also learned she had two very young children (under 3). All she spoke about the entire dinner was her divorce, and mostly only to him. She seemed eager to cut herself a lucrative deal and went on about the extra compensation she'd get for taking the children one extra day a week. She didn't even seem to notice that it wasn't exactly polite to go on and on with him privately while we tried to engage in conversation as a group. He obviously noticed and seemed a bit uncomfortable, trying to redirect her to converse with us all from time to time.
After she went home, we learned that she often dropped off the kids with him because she needed to work out to relieve all her stress. We spent several more days meeting up with her. At one point, she was outright rude to him. He seemed embarrassed and explained it to us as stress-related. By the time we left, we had witnessed enough. It's clear he's lonely, but it's almost as though she has cast a spell on him, warning him he would lead a very lonely life if it wasn't for her.
He's henpecked and he can't even seem to see the forest through the trees. My teenage children are very intuitive. They found her to be a totally self-consumed witch and wished they could plot a "Parent Trap" style camping trip to help him see the light! My husband and I found ourselves being very polite to her, but only out of courtesy to him, and now we believe that we left him with the impression we think she's a gem!
The more we contemplate the whole visit, the more concerned we get. In fact, I overheard her telling him that she could sell her (worthless) condo and move in with him and they could use the money from the sale to buy investment property on the beach. We've code named her Gold Digger and we need to know how to get him to ditch her ... fast ... before the divorce is finalized and she marches him off to Vegas! I suspect he'd be shocked if we were frank with him, and she seems the type who could easily manipulate the situation so that we are never welcome at his home again once he discloses to Gold Digger what we truly think of her.
He's obviously intelligent, but he's also still so vulnerable. He has all his deceased wife's photos and mementos around the house and he broke down several times when he spoke of her to us. We feel the "swooping in" is border line abusive at this point and if we say nothing, we'll regret not having made an attempt. Is there anything we can do? Should we just watch, jaws dropped, as Gold Digger moves in, kids and all?