Because they want to know what is the current financial situation of her and her family and based on that measure the amount of alimony. For example if she now have a good financial situation and her ex bad, the alimony would be lowered.
They take into account not only her earnings but also her partner's.
If she is applying for child maintenance, then the earnings of your son should not be taken into account.
If she is applying for maintenance for herself, it would be.
In the UK, the ex-spouse and the court can ask the new partner to reveal their financial circumstances - but only ask. If the new partner refuses, they cannot force him or her. It would be an idea to check if that is possible also in Poland. Your son should, I think, send a letter back saying that he prefers not to inform anybody of his earnings, and see what happens. Or, he should ask why. Or, he should go to a UK lawyer or Polish lawyer, depending where he is (you don't say if they are in the UK or Poland).
Either way, he shouldn't just answer with his earnings - he must take advice first. He might not have to reveal them. And of course the cost of raising his child with the Polish girl should also be taken into account.