IPhone from USA showing sim not valid

You or your friend will have to contact the carrier to get the phone unlocked. All major US carriers will unlock phones that have been fully paid for, but the process is different for each one. Smaller carriers (other than aT&T, Verizon, T-Mobile and Sprint) may refuse to unlock the phone.

Which would indicate the phone is locked to a previous cellular provider. Contact your friend to determine which carrier that is.