"Se" is indeed reflexive, but I think you may have the wrong idea of what a reflexive pronoun is: a reflexive pronoun is a pronoun that refers to the subject of the verb, and is used if the subject does an action to themselves.

Ex:

Ils se lavent (they wash themselves)

Compare with the use of a personal pronoun:

Ils les lavent (they wash them – not themselves)

In the sentence "elle (lui , leur , la) montre les photos", you should not select a relexive pronoun, as the subject is not showinng the pictures to herself. You need to select a personal pronoun that refers to a plural noun, and that has the function of indirect object. The only choice for that is indeed leur

elle leur montre les photos

It has indeed the same form as the possessive adjective leur. Just note that the personal pronoun leur is always plural but never takes an "s" at the end, whereas the possessive adjctive is "leur" in the singular, and "leurs" in the plural.

Il a vu leur maison (singular)

Il a vu leurs maisons (plural)

“leur” is a possessive adjective, but it is also an indirect object pronoun: it means “them”.

“She shows them …” would translate to “Elle leur montre …”

However, the first part the sentence (“ces est ses amies”) is incorrect.