That’s not how I read the OP.
The maintenance finishes Dec 2025 which has already been paid as a lump sum.
However DSS does not finish Uni for another 6 months (so summer 2026).
I think asking for another 12 months is cheeky but I don’t think asking for another 6 months necessarily is for the child to finish education.
Either way she should have planned for this.
But you need to consider the wider fallout.
If the OP and DH can afford it I would pay the extra 6 months but be crystal clear that is it and that whatever the repercussions mum needs to cut her cloth so to speak.
The OP say she works for herself and frankly this could mean anything from a low paying “vanity” project to a potentially successful business that may be struggling in the current economic climate.
Again, either way she needs to work out what she can afford long term and that may mean a change of job or downsizing her outgoings (including the mortgage either by refinancing or sale).
The flip side of course is what the OP and her DH can afford.
We were in a position where frankly keeping the relationship on a good footing was well worth the £££ and didn’t mean compromising our own (and our joint children’s) needs and wants. But I appreciate not everyone is in the same boat.