If this woman and her partner have been together for 5 years and have 2 children why on earth has she not asked him exactly how he earns his money and what is happening to it.
What kind of a relationship can she possibly have with this man, where such a long time into a relationship, candour about the basics of life are entirely missing?
In a situation like this I think you, your DH (and your DD) should assume that whatever he is doing is either dishonourable or dishonest until he explains what his business is, what his income is, and what it is being spent and provides proper (doumentary?) evidence to prove it. Asking to see his tax return would be a good start.
Is it possible that he has lost his job and is too ashamed to admit it and is, to use your expression, SEEMING to work long hours, and his only income is JSA?
Whatever the cause, I do not understand why you are paying for holidays and flights. Your DGC will not suffer because they do not jet off to Majorca or the Seychelles every summer. If you must help, limit it to basics: mortgage, possibly Council Tax and things for the children.
But at the root of the problem is your DD not being prepared to find out from her partner, what he does, how much it pays and where his money goes