Acrylic tends to stick to anything ( usually the clothing you want it to come out of the best...) if it's super thick the ridges may want to crack over time. Sometimes it sticks betterof it's roughed up a bit. She could lay the stencil on there trace it with a grease pencil or water soluble pencil and use some super fine sandpaper in the areas that will definitely be covered. But don't go past the edges or you'll see the difference. You might wanna find something with a similar type leather at a thrift store( even a small scrap for her to do some teats on ahead of time just to figure out the paint load and opacity) depending on the colors there may need to be an underpainting in order for the colors to b vibrant. Don't skimp on paint. Go with a brand like golden that has a high quality binder as well as a high pigment load. If you don't want paint ridges look into goldens fluid acrylic line. It's the same pigment load as the tubes but a thinner more liquid binder as opposed to the heavy body kind.
You could also go enamel if she's predomanantly an oil painter but it's trickier, more intensive clean up and less forgiving with mistakes
Because this is something that will be flexing on a regular basis you may have to have it touched up occasionally because most paints will still want to crack and flake at some point
There are some professional screen printing inks that are designed for printing on vinyl that may do better with the flex issue but they're designed to be laid down thinly with a screen, not painted with a brush. The brand is tw graphics. Google them If you wanna check it out the FAQ will give you an idea of what the inks will go on but thurs pricey ( the only brand I would recommend for this application don't just read silk screen ink an pick up some crappy speedball at the art supply store).
You could also go spray paint, but only if she's familiar with it. It still could crack but if it's a comfortable medium for her and she's used to it's tricks it's an option. Just make sure to get a kind that's multi purpose not just for metal and it should be more apt to behave
After the holiday I'll ask some of the peeps at work that I know are good at cross media application to see what to do to treat it before painting big it's super shiny it probably has a sealant on it to protect it which could make your life harder. This could be some sort of natural stuff ( some sort of wax like paste, think boot polish) or synthetic ( like scotch guard). Those can cause very different problems. Since it's new was there any kind of after care tag or did the salesman push any particular products for weatherproofing etc?
On a final note you may want to wear it around a bit before Ahe does this, it will start to break the new ness out of it and start to break down anycoatings and get the leather less creaky. That way by moving and flexing and taking it in different environments you're kinda micro roughing it up if you wanna skip sandpaper route. It will start to get used to how you're gonna wear it and you'll also get an idea of where it stresses the moat and probably think about placing the image so delicate areas don't fall into where it's wanting to crease
Also if you're a messenger or backpack wearer try to not do it once it's painted. That kind of every day abrasion will suck the life of the paint out quick. You might be able to seal it after it's done ( that's also on the things to ask about at work an I'll get back to you) but even if you do realize that your actions will always be putting stress on those areas and can sometimes be hard to touch up and still blend in with the original