If she paid for the COSTS, this is actually meaning : the film and its technical manipulation plus the paper (prints) and its technical manipulation but NOT the contend added by you exposing the emulsion, thus the WORK. The (intellectual) contend is the added value done by you wile working, and this is what not was paid for.
So, you can keep the contend and give the support (= emulsion) of that contend.
But by printing and giving the pictures, you actually already give the content made on the film, by this you de facto agreed to give the contend to...
And, as in this case, the contend is inseparable from the support, I think that there are (amongst others) two acceptable solutions :
Ore you duplicate the film (= costly) and give her the copy. Ore you scan (= less costly) the (best) images and give the original film bearing the images, to the bride (poor groom...).
And, in any case, write on the edge of each filmstrip your name with a fine permanent marker.
This is how, in sertain cases, it is done over here.
I know, this is a sticky (stinky) business...
Good luck,
Philippe