Well I suppose because she's only just realised that she potentially has that money to use.
Also you make it sound so simple, like it's a straight choice between feeling sad & ordering take away vs therapy.
She has an eating disorder, that makes it much much harder.
@HelpIsOverdue I'm sorry you're having such a horrendous time at the moment. While I don't have personal experience of eating disorders, I've read and listened to enough personal accounts of the experience of them to recognise the absolute torture they subject a person to and I'm so sorry you're suffering so much.
The amount you've spent has clearly shocked you and if you think you can use that shock to get help by paying for some private therapy then absolutely do so. I would also suggest you speak to your gp and share with then the impact on your finances - it might help push you up the queue or open up short term crisis support.
You don't gp into detail about your eating disorder or how it affects you so apologies if advice is trite...But if you're able to recognise your triggers for eating and why / how it helps you emotionally in the moment, then you may be able to replace some of the behaviours.
Some people overeat because they're numbing emotions or trying to ground themselves. There can be healthier ways to deal with strong emotion in the moment. A burst of exercise can help - a brisk stomp around the block or taking up a sport. There are grounding exercises online too, the 5 things, 4 thing, 3things, 2 thjngs, 1thjng is quite good. Sometimes a hot (not scalding shower) can help - or a cold one.
You could also try replacing some foods with others so that you consume less. Try to identify what it is about the foods you choose which help in the moment - are you sensory-seeking? - and look for an alternative. Eg crackers instead of crisps, posh hot chocolate instead of packs of chocolate biscuits.
I'm making some assumptions about the nature of your eating disorder and how it affects you, so apologies if I'm completely barking up the wrong tree.
What I would say is don't try to change everything all at once over night. Make a couple of manageable changes this week, and then some more the next.