There's not enough information presented to know for sure (you don't confirm for sure that the message came from a phone number instead of a cloud messaging account, what type of phone, etc.), but if they're both using Google Hangouts for messaging and both of them are signed into either's phone at the same time, or across other devices, all sorts of weird errors like this can happen.
Messages from the sender might appear instantly on their own phone but messages destined for the recipient will be received minutes or hours later.
Hangouts in particular has terrible sync across devices.