Settings you make in ACR for a Raw file are written to the separate XMP side car file. (The Camera Raw preferences has this set by default and to my opinion it should stay so, but the other option is to save them in a central database)
However, opening the same file in ACR and make new adjustments the previous settings are overwritten by the new ones. You can save them separately but that would be a hell of a job. If you want to be on the safe side you should make a proper back up of the whole folder (be sure to have the XMP files included, by default those are hidden files in Bridge) before someone else starts playing with the settings. However, you can always go back to the default version using the option in the little menu icon next to the name op f the panel (first panel is Basic). click on the icon and from the provided menu choose 'Reset Camera Raw Defaults' to go back to the originals.
I don't use jpeg in ACR very often but I believe ACR settings for jpeg and tiff are written in the XMP data in the file itself instead of a separate XMP file. You can revert to default or make new settings that also overwrite the older settings.
Making adjustments in ACR shows the results in Bridge (it uses the XMP data for the preview) but to make it visible in other apps you also have to save them in Photoshop itself to make the changes persistent so that mail and print reflect your settings.
So basically, let her play with the files and either chose the option to save the files (bottom left in ACR Window and choose the wanted provided options or open and save them in Photoshop to get the proper result. And if you want to be on the safe side let here use her own copies and keep the originals to your self.
Raw file settings are saved to XMP sidecars (or central database, if you're mad), unless you are working in DNG, in which case XMP is saved within the file.
TIFFs and JPEGs adjusted in Camera Raw have settings saved within the files. These adjustments are not visible outside Adobe products because they are just settings saved as metadata and do not change the images themselves. TIFF/JPEG settings are not saved as sidecars, so I don't know how you managed that in the past.
If you want multiple sets of settings for images, you should use the 'Snapshots' feature of Camera Raw, which has been around since about CS4 (I think). This enables you to save snapshots of your settings with images, and then change the settings, saving another snapshot, or reverting to the previous snapshot, and so on.
The only way to see the effects of your Camera Raw adjustments outside Adobe applications is to actually "render" the adjustments - i.e. finish the job. You can do this by opening the image in Photoshop and saving the result, OR you can use the Camera Raw "Save..." button at the bottom-left of the window.
ACR changes to JPG and TIF files are saved in the JPG or TIF files when you click the "Done Button". If you open them with anything other than Adobe software you will not see the changes since the changes have not been rendered. You see the changes in Bridge because it reads the ACR changes and renders them for the preview. You need to process them just like you would process any RAW file and save it as a JPG or TIF file and then the changes will be visible in the processed file with any software that will open the file - I recommend you save them in a different folder than the one that contains the JPG files you edited with ACR, or save them with a different file name. The JPG files you edited with ACR are your masters just like the RAW files you edit in ACR and you can open them with ACR anytime in the future and modify the changes since they are non destructive.
I process JPG files with ACR from my wife's camera since it does not have RAW file capability and keep the unrendered JPG files for the masters is case I want to make other modifications later. I also do this with files from my film scanner. I find it much easier to process using ACR with JPG and TIF files than with Photoshop.
The adjustments show up in bridge after she makes them in ACR, but don't show up (dark pic still dark) when I open the pic anywhere outside of Bridge (such as when we email it to another computer to check it or in windows). If we try to open in plain old CS5, ACR opens the file no matter what, which makes sense since the new info apparently isn't embedded into the file.
When she has made desired adjustments in acr to a jpeg she should do two different save operations from acr. First click "Save Image" button bottom left of acr screen, then the "Done" button at bottom right of acr screen. Select "Save in New Location" in the "Save Image" dialog box, and perhaps add a prefix or suffix to the file name so it won't be mixed up with the original jpeg later.
The jpeg produced by "Save Image" should have all the acr edits baked in and their effects in image appearance should show up in other jpeg-viewing software. The jpeg saved by "Done" will have the acr edits saved in it as acr parameters. They will show up in acr if you open that jpeg in acr but their effect will not be visible when that jpeg is opened by other jpeg viewing programs (as you observed).