There are many great alternatives. For example, we could have stories about a sleeping boy's dreams of becoming a superhero, full of exciting adventures in his dream world.
No, it is completely inappropriate and unethical. Any form of non - consensual sexual act, even if the friend is sleeping, is a serious violation of boundaries and basic respect for others.
One possible story could be that a sleeping boy's feet were peeking out from under the warm quilt. His little toes wiggled slightly in his sleep as if he was having a pleasant dream about running through a meadow. His feet were dirty from a day of playing outside, but that just showed how much fun he had.
One story could be that a sleeping boy dreams of flying over a magical land filled with talking animals. He soars above the treetops and has conversations with friendly birds and squirrels.
One could be about a boy and a girl who go on an adventure in a magical forest. They discover talking animals and a hidden waterfall. They have to work together to find their way back home before sunset. It's a story of friendship and exploration.
The idea of a 'cum on sleeping friend' contains inappropriate and unethical implications. Instead, we can have a story like when a sleeping friend has a bad dream, and you comfort them when they wake up, sharing warm words and hugs.