have you tried putting cherrios in the toilet?
I have daughters and I just got my 2 year old trained completely. They were easy though. I went out and we bought new princess panties, and got rid of the diapers and neither one had an accident.
One important thing to remember is that your daughter needs to keep her son at home all the time till he is trained. Taking him out places may hurt the issue.
Is she using the big boy potty for him or the little kids potty? She might try switching to the other and see if that helps.
Your daughter needs to make sure that if he drinks something that within a short period of time she puts him on the potty, and she has to keep doing this. Boys can be hard to train, but there are books that your daughter can put by the potty about potty training so that he can read.
Have you tried the good ol board of stickers? Get some cool spider man stickers for him and let him put it on the board when he goes to the potty.
kids are not going to potty train until they are ready to kids do things at their own pace my daughter was potty trained at age 2 tell her not to lose her patience and he will do it when he is ready to just keep working with him.
My husband trained our son by turning it into a competition. We have a nephew that is a few months younger, my husband would pretend to be on the phone with my nephew, and tell my son that he was on the potty also. He would make it a race to see who could poop the quickest. For pee - with a boy, we made it fun. We let him pee on trees and rock walls, and off the deck. Thought it was the funnest thing. He was pee trained at 3 yrs - but poop took until 3 1/2. It will happen - if she looses patience, then he is seeing it as a challenge and pushing her buttons. Good luck!