Can she kiss her female friend on the cheek when she is fasting?

There is nothing wrong with a girl kissing her female
friend on the cheek when she is fasting, so long as the intention behind
this kiss is to show love and friendship, and not physical desire. 

Shaykh Muhammad ibn ‘Uthaymeen (may Allah have mercy on him)
said: 

Kissing (on the part of one who is fasting) is divided into
three categories: 

1.

That which is not accompanied by any desire whatsoever, such
as when a person kisses his small children, or kissing someone who has
returned from a journey, and the like. This does not have any effect on the
fast. 

2.

That which provokes desire (such as if a man kisses his
wife), but there is no danger of the fast being invalidated by emitting
maniy (semen). The view of Imam Ahmad is that kissing is makrooh in this
case. 

3.

Where there is the fear that the fast will be spoiled by
emission of maniy. This is haraam if it is thought that maniy will be
emitted, as in the case of a young man whose desire is strong and who loves
his wife very much. In this case there is that risk if he kisses his wife in
this situation. In this case we say that it is haraam for him to kiss his
wife, because he is exposing his fast to the risk of being rendered invalid.

 With regard to the first category, there is no doubt that it
is permissible, because the basic principle is that things are permissible
unless proof is established that they are not permitted. As for the third
category, there is no doubt that it is haraam. 

With regard to the second category, which is that of a person
whose desire will be provoked if he kisses his wife, but he is confident
that he can control himself, the correct view is that it is not makrooh in
his case, and there is nothing wrong with it, because the Prophet (blessings
and peace of Allah be upon him) used to kiss his wife when he was fasting.

Narrated by al-Bukhaari (1927) and Muslim (1106) 

Therefore the correct view is that with regard to the one who
is fasting, kissing is divided into two categories only: that which is
permissible and that which is prohibited if the person is not certain that
he will not render his fast invalid. 

The category that is permissible may be further divided into
two categories:

1.    

cases where kissing will not provoke desire at all;

2.    

cases where it will provoke desire, but he is confident
that he will not render his fast invalid. 

As for actions other than kissing that may be precursors to
intercourse, such as embracing and the like, the ruling on those actions is
the same as the ruling on kissing, and there is no differentiation. 

End quote from ash-Sharh al-Mumti‘ (6/426-429) 

And Allah knows best.