My now 6 year daughter did exactly the same thing last school year. After 2 years of preschool with no separation problems in the middle of January last year she suddenly had trouble getting on the school bus. She dug her feet into the street and broke down crying. I had to take her to school for 2 months or longer even though her 2 sisters were on the bus with her.
She cried when I dropped her off at school. Sometimes she would let me walk her in, sometimes she would let her oldest (7th grade) sister take her in. Always crying. On bad days she wouldn't separate at the front door and I took her into the morning assembly and would sometimes sit by her until the classes went to their homerooms. Often the teacher would have to take her hand and she cried.
I thought that maybe something had happened on the school bus or that an older child or teacher had said something to her that had hurt her feelings- she's super sensitive to criticism. But nothing ever was confirmed or discovered.
She started crying in her classroom too and sometimes in some of her special/extra classes. I talked to her teacher and finally talked to her principal. The principal talked to her too but no discovered any more than we did.
I took her to the peditrician's office because when she was nervous and didn't want to go to school she said that her stomach hurt. A physical problem was ruled out.
Her principal suggested that maybe she just couldn't express what was wrong and she thought that if the problem was serious enough that my daughter would confide in someone, probably one of her sisters. We told her that it is okay to cry and that she could carry extra tissues around if she needed them. Her homeroom teacher told the other kids that it is okay to cry and basically not to make a big deal of it. The kids would just go on with their work.
She did carry extra tissues and eventually after 2 1/2 mos or so she got back on the school bus and has never looked back. No problems this year. I really think that it was a maturity issue and I did notice at the beginning of this year that she asks if I am off of work that day. One day when I was off she started to say that she didn't feel well but I told her that it is her job to go to school, just like it's my job to go to work usually. If she's really sick at school, I told her to go to the school office/nurse. Pretty smart.
Good luck- tell your friend to be patient b/c it might just be a stage/maturity issue. I never went through that with either of her 2 sisters though.