Sounds like it might be a network issue. But its hard in these cases to determine if its AT&T's network or on Softbank's side; and they will likely blame each other. Try calling AT&T customer service and explain your problem, and see what they can do. Ask you friend to do the same on her side (call Softbank), and hope for the best.
I am having the same problem, delayed text, if i even ever get them. And my signal randomally will just drop for no reason in places i get great signal. I have contacted AT&T a few times but they have yet to do anything about it.
are you using the email to sms/mms address to send or get the messages? if so try messaging her directly and have her reply or message you directly and sé if there is any type of delay (technically sms/mms delivery to the recipient can take up to 24 hours domesticly or longer for international usage).