It's too bad that she is devastated because she is definitely prego!
Hopefully she can come to terms with it and soon. Having an 8 year old is good in a sense that she doesn't have two little ones to chase around and the older child can be a great help.
The only time you can have a positive result and find out later you're not is if you miscarry.
Tell her to join in here if she needs reassurance and support.