Doesn’t seem to be Sumatra’s fault, but why don’t you run a quick test to check? With TB closed completely on both systems (not even running in the background, minimized to the tray etc.), on your wife’s laptop back up her TB profile folder to some other location and delete it. Copy over your profile folder’s contents using the instructions here, then launch TB. Now TB on your wife’s laptop should display your mail account and ideally behave the exact same way as on your laptop. Can you open PDFs in Edit mode now on her laptop?
If yes, that means it’s a problem with your wife’s TB profile. If no (i.e. your profile on her laptop still exhibits the same issue), it means the problem lies elsewhere. If Sumatra’s settings are also exactly the same on both systems, then perhaps there’s some difference in the registry settings related to PDFs.