Tricky situation

Yes, I think this is the best option. Make it very clear at the start of the evening to the restaurant that you want two separate bills, and he's not to add anything to your bill, that you're not paying for his drinks etc. At the end of the meal, you get your bill, he gets his. If the waiting staff add anything of his to your bill, ask them to remove it.

I haven’t read the whole thread (apologies) but in my opinion, your response depends on what you are trying to achieve from this tricky situation:

1. do you want your dearest DC to enjoy their graduation celebrations on the day?
2. or do you want to teach your ex-H a lesson and show him (and to an extent your DCs) that you will not be putting up with his antics?

To me it’s clear - you agreed for the ex-H to attend the meal celebrations at the DC’s request, in a bid for your DC to be happy to have both parents there for their big day.

Your husband is not likely to start behaving differently than what you know, so that means he is likely not going to pay for his share of the meal/drinks whatever way you look at it. Yes, you can tell him in advance, or the restaurant on the day, or come up with other ways to make him embarrassed to pay, but really - you should be prepared that you are going to cover his costs as well if you want to avoid spoiling your DC’s big day celebrations.

On this occasion and the fact that you already indicated that you are fine with him attending and you are organising the whole lot, I would be the bigger person and focus my energy on my DCs only. And for the future, I will consider having two meal celebrations so that I don’t put yourself in another tricky situation like this.

Look up average costs and pop him a message before ‘did you want to send me the money now or we split the bill at the table?’

Corner him at the start and say thank you for going 50/50 with me on the evening, the children we’re worried that I might have to pay for everything. Nothing worse than meanness is there?

I’d sit there hoping that the waiting staff have that underlying expectation that ‘the man’ is footing the bill and see how he deals with it. When he gets handed the bill say “oh that’s a lovely gesture for dc’s birthday. Don’t worry about me, I’ll pay for myself, you just cover yourself and the children ☺️”

I would tell Dc who invited him
to make it clear . Also announce when ordering you want a bill for you and Dc . Will make it crystal clear

I think I’d be asking DC why they want their ex-DF at a celebration which will be awkward/cool/ or worse when drink taken a stand-up row. It won’t be a ‘celebration’.
Has DC some daft idea that the ex-parents will suddenly be able to ‘get on’.
I think an honest conversation is needed with DC

When the bill comes say "I agreed to pay mine and the kids share if you get the rest?" And quickly put whatever in the card machine/on the table, and have a long conversation with one of the kids while he sorts the rest out if he threatens calculators etc. So boring.
Grab your handbag and casually get up after 5 minutes...
Just make it look easy.

Be careful, he might say he'll pay his share but then might scarper leaving you with the whole bill, or the option of a confrontation with the staff,all just because he's petty... the tables booked in your name. They'll expect somebody to pay it!

Either ask for your bill to be separate at the start or when they give you the bill tell him to pay for his and you'll pay the rest

Yes - I agree, tell him in advance how the split will work. If he is anything like my ex it will mean he won’t turn up.

Is this going to be a 'fun meal/celebration' anyway, but, obviously you have to go ahead with this as your DC has requested it.

Many parents who aren't together get on or are at least civil and can and do go out for dinner when there is a special occasion BUT, it doesn't sound like this is going to be the celebratory meal that your child is hoping for with laughs and chatting whilst having dinner altogether as a family.

Sounds like your ex is awful and sounds like you have a great relationship with your children and have been a great Mum so it's a shame that instead of worrying about 'what to wear' you are having to worry about 'your ex'.

Maybe at a later date you can organise a 'celebratory' shindig with all the people that have been involved/family/DC's friends etc without the worry of your ex.

I know this probably wont happen but, is there any danger of him just 'picking up the whole bill' as that is exactly what my ex would do!?

Well he would not be invited if it were my ex. I would never have this problem.

All of this.

Just tell him discreetly you can pay for yourself OK.

So your 'lovely' child asked you if you were ok with your selfish, financially-abusive ex-DH attending their graduation ceremony? and you said yes. Fair enough. But does your ex really need to come along to the meal as well? Why would your DC want to put you through that? Why can't they meet up with their dad eg for brunch (or just coffee/ a drink) before the ceremony? Sitting in a large hall watching your child go up on-stage to collect their certificate is very different to sitting around a relatively small dinner table together, and presumably it'd be a lot easier to deal with him there?

I would say at the beginning ( with a big smile on your face ). Would you like to go 50/50 or would you prefer to just pay for yourself ?
Then ask the waiter for a separate bill for his share .

I agree with asking for a split bill, just make sure he pays for all his own drinks too.

Nice goading. Sell it somewhere else.

@Littlebluebellwoods as per my post I suggested giving DC money in advance if splitting the bill is the way the OP wants to go. Bill arrives, host announces £X so split Y ways we all chip in £Z. Money goes in the pot. Very common. If card payments only at the venue, whoever puts their card down takes the cash pot.

Excellent idea.

DC hasn't forced anything. OP says DC asked and OP said it was OK.

What silliness - and arrogance to boot - to try and tell a mother that she's wrong about the character of her adult child and that you, a complete stranger on an internet forum, know better.

I think it is more than reasonable to put the explanation of financial expectations on your adult child who wants him there then.
Just reiterate at the restaurant before ordering you will need separate bills.
Remember if he " forgets his wallet " the drama is not caused by you, you've already been gracious enough to agree having him there.