Now, from my understanding, the word for 'friends' (I'm assuming it refers to singular and plural) is
Tomodachi and is indescriminate of gender, so in this case the text in Minato's speech bubble would 've read as somthing like
'Naruto no tomodachi' or 'Naruto's friend(s)' (if anyone knows further about this and wishes to correct me, feel free

).
Kanojo, is to refer to a female in the 3rd person i.e 'her' or 'she'. However, it has also taken on the meaning of 'girlfriend' i.e. female romantic partner. An example -
'Ore no kanojo wa utsukushii desu'(let me know if that's right) literally means 'My 'her' is beautiful', with 'kanajo' meaning girlfriend in this case.
Now, on to the panel. I'm not sure how to post this as an image, so I'll provide the
link to the panel in it's raw form. (recommend opening this in a seperate window so you can have both pages on screen).
Tomodachi is written in Kanji as
友達, and in Hiragana as
ともだち.
Kanojo is written in Kanji as
彼女, and in Hiragana as
かのじょ.
From looking at the raw panel, one can quite clearly see that the text does NOT say
'Naruto no tomodachi', it says
'Naruto no kanojo', literally meaning 'Naruto's 'her/she', with kanojo having the same meaning as in my example. Both versions of 'kanojo' (Kanji and Hiragana) are also provided in Minato's speach bubble by Kishimoto (highlighted by myself in the red box). If one zooms in, one should be able to clearly see the 'no' (
の) in 'kanojo'.
In the case of this phrase, there is no other meaning to it other than Minato questioning if Sakura is Naruto's romantic partner, his 'girlfriend'. If Minato was seriously asking whether she was Naruto's 'friend' i.e. a person with whom Naruto is closely familiar with, then surely Kishi would've had Minato say
Tomodachi (which, as I said I am to belive, is refers to both males and females), instead of
Kanojo, because lets be honest, it doesn't sound right, does it? Why would Minato go out of his way to ask Sakura if she was a close aquaintance of Naruto who just happens to be female - kind of stating the obvious, isn't it?