Wwyd if your partner did this….

Her poor kids, coming second to an abusive arsehole because their mum values her relationship with him more than their wellbeing.

If he's confident and comfortable enough to do something like this in front her her, and have no consequences, god knows what else he will do.

I would report this to her school so the safeguarding lead can deal with it. She might be desperate to speak to someone about her home life.

Poor kids.

If you are worried, and I would be myself, I would share the information. If you aren't comfortable sharing directly with your local children's services, you can share with the child's school who can gently discuss and report. The best thing you can do is reinforce to your friend and her child that it's not acceptable and be a safe space for the child. What you've described has made my Spidey sense tingle, I'd be inclined to suggest your friend applies for a Claire's law disclosure. It can be tempting to be annoyed but if you can remain firm but there it prevents your friend being isolated which if this man is abusive, will be his aim

Tell her not to be surprised when her DC are adults if she doesn’t see them again.

I would report that incident to social services and I would tell her I am reporting it.

Confront her with what she's allowing to happen. Ask her what mother allows this ?

She's a shit mother. Putting a man over her child. Why are some women so desperate for a man.

We will all have our own opinions on what we will tolerate. Like you, I wouldn't.

You say he has a temper, so it might be a sad fact that this is minor in her eyes, compared to what else she is tolerating.

So, to address this from two angles since you mentioned reporting:

Me personally as a mother: totally unacceptable, what an awful person! What else has he done?

Me under my professional hat... What were the circumstances? Was the drink thrown at the child or in the direction of? Does this man have a history of DV/DA? What is the history of the family? Do the children have a relationship with dad? Wider family? What is going on personally and what was the build up? What are the housing and finances of the family? Why does mum think it's acceptable for him to remain? How will mum safeguard the children from further harm? Do the children see dad? What is the school's view on the children's presentation?

Obviously it's not acceptable behaviour, but a SWs job isn't to tell people what to do, so if she isn't going to bin him off it's not as simple as the SW telling her to.