I am with @Millytante here.
The idea that he will suddenly change his odious grasping mean character is so beyond deluded, so OP please disabuse yourself of this vain hope.
Not a chance or hope of this happening.
He is who he is and he clearly has been dragged up.
I can completely understand your confusion.
It is a good idea to open any conversation from a position of very faux concern about the meanness, money issues, concern that she isn't being taken advantage of financially???.
You know his salary?
Where is all HIS money going?
It is very normal to express confusion and shock that people would visit in the way they did and how they behaved.
Faux confusion can be a very very effective tool.
Do it in stages.....if there is a suggestion of a wfh visit, knock it on the head completely, don't entertain it.
If she mentions a summer visit with his parents, which he undoubtedly will, why wouldn't he, you could say the below....
(Shock and confusion at the manner of their visit and how they behaved.
And then of course you can be very faux apologetic when you say "Darling we really are not a free guest house for people like this, it was a really exhausting expensive week. We will put it behind us, but it won't be repeated, I'm sure you can understand".)
She can visit again with the boyfriend BUT with plain simple meals that he doesn't like.
Absolutely no going to pubs and restaurants and paying for anyone but yourselves, spelt out clearly.
Personally I would tell them you are doing a liver cleanse so no alcohol being served.
So what if she is shocked, embarrassed or upset?
That is for her to figure out.
You have boundaries and are not mugs.
It is perfectly reasonable if challenged on this to say sweetly to her "Darling, we have always been very generous hospitable people but we will not be taken advantage of by anyone".
Denial tells her this is acceptable.
Sweetly telling her it isn't is absolutely your right.
If your daughter wants to flouce off, that is on her.
But allowing this to continue will not do you any good long term.
She is an adult and she needs to be treated like one, but you showing her respect for her and yourself by being sweetly honest is the way to go IMO.