Mumofellie welcome to the thread, I agree with the others that rather than squeezing in another snack they'll want to increase what she's already eating. Are you adding butter/double cream/fats to her food as this is the easiest way to get more in without them noticing.
For example my dd would have porridge for breakfast made with double cream which worked out around 800 calories (dd obviously had no idea re the double cream!)
I also added cream and butter to whatever she had for lunch and dinner as well as looked for opportunities to add in extra bread or calories any which way I could.
If you give me an idea of what she's currently eating I can help with how to increase its calorie value.
Wooden I'd be begging the team to section him tbh, he is so ill and unless he starts eating more his vital organs will start to fail. If you notice blue lips, coldness that he can't get rid off, cool/pale limbs or he mentions his heart feels funny ring 999. If he refuses food I'd take him to a&e or ring 999.
Can't blind weighing was our only way round this. Dd also had a totally unrealistic weight in mind and said she'd kill herself if she went over this. I responded with well you definitely can't know your weight then!
Now she's stable she recognises knowing her weight is harmful and actively avoids knowing it.