I've not been in your situation, but my daughter has anorexia and last year lost a lot of weight and became very ill.
Apart from her weight, other things to look at are her blood pressure and resting pulse. Do her hands feel cold? Is her hair falling out? Is she more obviously tetchy? Forgetful?
Will she go to the GP? Whilst it sounds like a loss of appetite rather than eating disorder, the outcome of restricting intake is the same and she needs support and regular physical health monitoring.
Calories in and 2 litres of fluids a day is necessary. 1500 calories as a minimum so she stops losing weight. 2500 is recommended by eating disorder services as the level for weight restoration.
By the sounds of what she's eating, it's "safe" food - bland, consistent, easy to consume. Offer her something every 2-3 hours - essentially 6 meals a day (3 meals, 3 snacks but if 6 x 300 calorie options work better for her, that's ok).
Look for any ways of adding calories - cream in coffee, butter on toast. A bowl of nuts for her to graze on. You want high calorie, low volume. If she's drinking but not eating, look for milky drinks and fruit juice. If you can get her to a GP, ask about a prescription for Fortisip.
Would she take a multivitamin/iron supplement as well? Just to alleviate any pressure around nutritional value of food at this point.
Beat is the eating disorder charity and they have a wonderful helpline. They can't give medical advice but can signpost you to support.
I hope things improve quickly x