SMS intercepted and edited before arriving?

I'm hoping someone here can help me with something. The other day I received a text message from someone and the latter half of it was odd and didn't have anything to do with the conversation.

When I asked the person who sent it they said they never sent it... even though it was part of the original message bubble that i received from them. I even looked at their phone and the part in question was not there on their phone.

So i'm wondering: is it possible for someone to edit out/delete part of their sent sms? Or is there software out there where someone could possibly intercept a message, ad to it, and then pass it along so only the receiver has the "additional" parts?

I'm using a samsung galaxy ace 2 an they have a samsung 3. Sorry if this is vague or not specific enough technically... just confused and don't know much about this stuff.