My iMessages will not send to my friends, but I can send and receive (my friends included) to everyone else I have a small bit of a problem. My girlfriends Apple ID is currently linked to her school issue iPad and her personal iPhone. After awhile of iMessaging back and forth on the iPad, my iMessages will no longer go through on her phone and never give the delivered tag. However, I can message everyone else and even receive her iMessages; the only thing I cannot do it reply to her iMessages.

She does not have access to the iPad anymore as she has graduated and had to turn it in. 

Anything will help! Thanks!
Your girlfriend's Account preferences have all her messages going to her iPad. 

On her phone, she should go to Settings -> Messages -> Send and Receive and make sure that all the addresses under "You can be reached by " are checked.