Well someone could link into her signal and find the exact coding of the phone, copy it and use it to make calls.
But then how would they know all of your friends names.
And of course some phones have online pages where you can make texts from it, ( like ATT) so someone could have hacked her security and did it.
But it would involve someone who either has the equipment or programs, who wants to cause trouble.
So is it possible, yes, is it very likely or does it happen a lot, no, what does happen people are sending texts and hits a wrong name and sends it to someone they did not want to receive it