I have worked with many children and have seen this more often than people think. Some do not want to take time to potty and others are just lazy but all of them need love and support. My middle son wet his bed every night until one uroligist put him on a nose spray. He was 11 or 12 by that time so you can imagine the problems that caused.My opinion is that today we have to let the child decide when they are ready to come out of diapers instead of encouraging them when the parent sees the ready signs. I have 8 children and they were all completely trained at a different time in their lives.
As for what can you do? Some say to stay out of it but, I am one grandma that speaks out when I see one of mine being mentally abused. Encourage you daughter to stand up for him.
Also as someone else said; there is nothing unusual about a boy this age playing by himself at day care. I am not an expert but I have worked with children for 40yrs or more and have a cerificate in Early Childhood
We all know though that this is just my thought.