Ouch!
My XH left me for an affair after 11 years of marriage with a 1 yr old and disabled 3 yr old so I do get the emotion on this.
I think it is wrong to berate a young child for showing compassion and welcoming into HIS family. The step mum is legally and morally a part of his life and family. He no doubt will spend a chunk of his life in her company.
What does mum want him to be sad, bitter, angry at each one of his access visits? Of course not.
She has no doubt taught him what he knows about love and reciprocation. How little gestures or expressions of appreciation mean a lot. She has taught him to love with an open heart. These are wonderful lessons to have passed on.
She needs to apologise to her DS before this becomes a major issue. She could explain she'd never really thought about his step mum as a mum but she realises thats wrong and she's proud of what a lovely thoughtful boy he is becoming.
The reality is that this isn't a well thought out plan more a case of a teacher saying something PC like you can make cards for Granny or your carer etc because not all children are lucky enough to have their mums around let alone two mums to care for them. Her DS probably finished one quickly so went on to make another.
Its lovely that her DS is so open with her that he proudly showed off his artwork. Its lovely that he is a thoughtful boy. Its lovely that he talks about what he's been doing with her. She needs to gain strength from these points and learn the smile and nod when little sharp knives of emotion catch her off guard like this. She will always be the no.1 Mummy. No one, but her, can take away or undo all the love and strength she has instilled in her DC. That compassion he's showed in making a card has come from her.
There are so many potential ouch points especially when you're dumped with young DC. I love the return visits comments like 'Daddys buying us a garden full of toys at his shinny big new house'. With his wage and hers coming in whilst we struggle to turn the heating on and trawl car boots to put things by for Christmas.
She needs an outlet for the anger and to learn the smile nod, go to another room and punch a pillow. Come on here and vent, phone you when he's out of earshot and vent.