Help me understand PLEASE

my GF has ED, and on her recovery plan is Tuna, and lots of other things like Vanilla malt milk and butter, she hasnt had bad interactions with these foods yet they still are extremely detrimental to her mental health when they're in the house or being consumed by anyone.

I really like Tuna and always have, and when I wanted to get some because I have been wanting it for so long she says I can't for the reasons I've explained above, I'm extremely uneducated on eating disorders and how or why it affects her when she hasnt had a negative interaction.

I ask her why and she doesn't seem to have a logical explanation, can you please help me understand why this is a problem and how Eating disorders work.