Not sure if it still works the same nowadays (god I sound old!) but have you replaced the first zero in your friend's mobile number with the international dialling code for the USA (+1)?
Conversely, to 'dial' into the UK you need to replace the zero with +44.
".....country code for US is 1 and UK is 44.
Standard format for international SMS is generally :
(plus sign)(country code)(area code if any)(mobile no)
so to text US : +1 XXX XXXXXXX
area code 3 digit
text to UK : +44 XXX XXXXXX
area code 2/3 digit
please note if you are given area code beginning with '0'
you may omit the zero....."