Absolutely it's up to the OP, but that doesn't mean it's irrelevant.
If someone is shacked up with Elon Musk, then it would be tight of him not to contribute, and unreasonable for the bursary to continue.
If someone earns enough that the bursary wouldn't be offered, it it wouldn't be realistic for her to contribute either, then not only wouldn't she be tight, but it would be unreasonable for her partner to even propose it.
Of course she's not obliged to pay it, but like other decisions in life, it's going to have repercussions. It's clear that she doesn't see them as a family unit, and that's fair enough, but if he does, then it's going to cause tension.
If the change in school is going to have a devastating effect on the son and a real change in his lifetime prospects, and in reality the money would make negligible difference to the OP, then that is relevant.