Well I think there's some good advice on here. My DD has an Eating disorder & is currently in recovery - it never really goes away a bit like alcoholism. It went on for 15 years & in my view she's lucky to have survived. She spent many years in inpatient treatment (no exaggeration) but I'm sorry to say it didn't really help.
What did help was a private dietician, private counselling from an nationwide organisation called SWEDA & day & night care from family for a few months.
In my experience even the people who are supposed to know what to do, don't really.
I suspect that your partner had issues as a teen with food however fleeting, usually it rears it's head again at stressful times. Something has re triggered it & I'd say try to look at this time as an opportunity to help her look at what she's struggling with underneath her eating issues.
It's people who are prone to obsessional compulsive disorder, depression & anxiety. I think everyone I've met who has developed an eating disorder has a combination of these, and it needs to be picked apart to try to work out how to deal with it.
Find out what she wants to do, what she'd like to work on, and how she wants to be helped & try to get that as matched as possible to what support you can get for her. I say this because all too often it's like they push people down a route that isn't helpful because it's not really what the unwell person wants & will never really work, perpetuating the illness. And then they become "revolving door" patients, in & out of hospital & inpatient facilities. This is how my DD was for years & it didn't help. It just made her angry because she wanted them to do something different, care, some individual help & support, she wanted to be seen & this was sorely lacking.
At the time she didn't know how to say this, just kept saying she wanted to die, but she can say now that she didn't really want to die, she just wanted people around her to do something different.
So I'd say really listen to her, be mindful of being sucked into "treatment" because unless she's given space & support to understand what she's doing, why & what she really wants, treatment has much less chance of helping.