My daughter-in-law dominates my son

Well said Peasblossom, the person he needs to be talking to is his wife!!

Yes I would say the same about a daughters behaviour.

Yes, it would.

No I didn’t express it kindly.

Let me put it another way. There is a pattern to his behaviour which is that faced with a problem, he turns to other people to sort it out.
The fact that the OP is seeking ways for her to make things better for him is also a pattern of behaviour that encourages his behaviour of dependence.

Very little behaviour is one-sided. There is a triangle (quadrangle?) of behaviour here that doesn’t encourage adult growth and responsibility.

Not very kind Peasblossom. Is a son not allowed to discuss a problem that is making him unhappy with his parents? If the son was a daughter who was talking a problem over with her parents would that be 'whining to mummy and daddy' too?

Well there we are. She’s domineering, he’s a grown man who, instead of sorting out his own relationships goes off whining to mummy and daddy to sort it out for him.

And mummy and daddy are ready to step in and “help”.

He’s unhappy. I don’t expect she’s very happy either.

The best thing you can do is to be there for him and give him opportunities, like the lunch , to talk. Just being able to talk out loud may be enough for him to think about what to do next. Don’t offer any advice on their relationship or solutions, you want him to realise his own strengths. If he found the counselling useful he might want to have some for himself. It might be a situation of emotional abuse, or it might be that he can learn different ways of communicating so that he is more equal. You are doing the right thing by being there for him and not dismissing his feelings.
And she’s not a great mother if she modelling bullying.

He IS very unhappy. He had lunch with my husband (his father) at the weekend, and told him so in great detail. They talked for hours. After a couples counselling session (quite some time ago, I think), she came out of it saying "Well that was all ridiculous"

How do you know your son is unhappy? Some men like their wives to make the decisions, maybe he’s one of them. Has he asked for your advice or opinions? She must have some good qualities if he married her and you say she’s a great mother. You shouldn’t say anything to him or her and keep your opinions to yourself - you don’t know everything that goes on in a relationship.

No. Simply no, I’m afraid, without making things a whole lot worse than you already think they are.