Thanks momtofour, the mom is one of DS's former teachers so she knows him really well. He must have talked to her last week at school about what to expect today because this morning he seemed to have a good idea of what would be going on. I think for the most part his role today is to hang out in a really fun rec room with his friend before and after the procedure and be nice. From the pics the mom has sent me so far, the boys seem to be doing well : )

An aside on these kids - in K, DS was really into the immune system and would talk to his friends about various white blood cells all the time. When his friend was diagnosed, his mom said it was really comforting for her son to already know some of terminology he picked up from DS.