Partners child is 20 going to uni how does it work ?
The DC is assessed for the discretionary part of the loan on household income (unless over 25, or able to demonstrate they are living independently of parents, neither of which seem to apply here)
Tuition fees are included in full for everyone in the loan.
Living costs are on a sliding scale and parental contribution covers the difference between loan given and the likely cost (ie at least up to full loan, and possibly beyond especially if they want to study in a more expensive place)
So of course the DParents, whether still together or not, should continue to support the DC. Until the end of full time education is a very normal situation, payments can of course go directly to the DC
Even though you mention the DC is 20, so has presumably taken a gap year and/or was disinclined to start because of covid, I think he should support DC practically (including financially) simply because it's the right thing to do.
I do realise that for some people it's totally impossible (if you're living hand to mouth) but it should still be the aim.