Interesting to me you asked this b/c awhile back I was interested in doing something similar...the pre-made X number of spaces for photos in a frame. I was interested in stamping on the mat material that surrounds the pictures...maybe to make presents for family or a child's room. I never did get to it so I have no idea what inks will work on the mat yet.
How flattering she likes your work this much!
My suggestions:
1) get the framed piece that she wants to use-and try working on it like a practice run. If it is matted...you could ask at an art store what inks/paints/adhesives they think will work on the mat. You could maybe just get plain uncut mat material to practice on-that might be the cheapest way to go. But you want the same kind as in her thing-if she has it already ask to borrow it and take it in to show them. As we know, inks etc react differently to different surfaces. You might have to use pigments and heat dry it. Or Stazon. You may be able to do water based inks for blending...who knows?
If you can work this out...you might be able to sell these too.
2) the concern I have is not only to work with copies-excellent point!-I expect copy houses might be to do that on photopaper directly or scan first.
But you dont want to overwhelm the pictures with a lot of stuff visually. They could end up looking at a glance like DP in a busy field. One idea might be to have a black border you dont cross to maintain the image visibility.
I totally agree with the others....ask her to pick the pieces of your work she is loving to begin with and then really nail down what she envisions for this project. If she has seen something online she likes, get the link so you can see it.
Collage can mean many things to many people. Some see it as paper layers and minimal embellishments. Some see it as whole mix of materials.
In which case it isnt just how to stick things to the mat...it is which adhesives to use to stick the different materials to each other. Which may not be any big deal.
Last part...if she is not using a shadow box..you are not going to have a lot of play dimensionally which could curtail what materials you can use.
Just some thoughts.