Can a phone be hacked? Really in need of help.

[yooperbug]

I'm posting this question for a friend that has a LG Spectrum. I really need help understanding how someone can have a text CONVERSATION with someone and make it look like all the text messages came from his phone number. I also really need to know how to prove that those text messages didn't come from him. There was supposedly a whole back and forth conversation that took place and all the messages had my friend's number but my friend isn't the one that had the conversation. Thanks in advance for any help I can get.

 

[Lykarsis]

Only thing I can think of is that someone activated another phone on his SIM card then put his back in his phone before he noticed. Or someone went through a lot of trouble and cloned his SIM.