This is probably something to do with how Sense-based ROMs handle background applications. Incredibly often, your activity will be killed while in the background because it's consuming too many resources - at least, that's how the system views it. This is not normal behavior and is really up to OEMs (HTC, specifically) to fix.
To explain the behavior that you see, when she restarted her device, it cleared all background applications so that your app had room. Gallery for Sense must be much more resource-hungry than Dropbox's image selection, which is what permits your app to either stay or be killed in the background - if more resources aren't needed (Dropbox) it's left alone and functions as intended. If they are (gallery) it's killed.
