So, friendship here is a noun in the singular form, which calls for the use of the third person singular, is, not are. Friendship between girls is so beautiful. Unless you’re referring to the girls being beautiful, in which case you’re right in both cases.
Thank you for enjoying Pedant’s Grammar Comment.
-BIG in the ass.
-My left foot in the ass
-To kill a mockingbird in the ass
-Wolf. In the ass.
-A new hope in the ass.
-Mallrats in the ass (on topic as well).
-Stigmata in the ass.
-Snatch in the ass.
-Lock, stock and two smoking barrels in the ass.
-American Grafitti in the ass.
-The exorcist in the ass.
-Clueless in the ass