Last week I looked after a friend's DC for an afternoon as she is going somewhere her DC can't.
I offered. No problem there
She was given a packed lunch when her mum dropped her off. I said there was no need but her mum just said 'oh no, honestly, it's fine'
Assuming fussy eater etc I didn't question it and just let her have her packed lunch. We had some lunch alongside her
She had finished what she had and asked if she could try what I was eating. I said no problem, of course. And gave her a plate. I said eat what you like and don't worry about leaving what you don't
She ate it all. And for all of us afterwards, I had a cake. It was a red velvet cake from Costco. It is really yum and a nice treat
She had a slice and then another slice because she enjoyed it. She left a bit of the additional slice. Again, a non issue
I got a harshly worded text from her mum that said 'Hi, Y said you gave her additional food after her lunch provided. In addition to this, she had gorged on a lot of cake too? Please in future can you stick to what I provide'
AIBU to think what's the big issue? Have I overstepped some mark I didn't know about 
The little girl is 7. My own daughter is 2