Q: My girlfriend and I use the same iCloud account. Can she use the Find My iPhone service to erase everything on my phone?
A: Assuming that she knows the password to the shared iCloud account, then the short answer is yes. Basically, anybody with your iCloud account name and password can log into the Find My iPhone feature at iCloud.com or use the Find My iPhone iOS app and access the same features that you can; since it’s all one Apple ID, there is no way of knowing who is actually logged in with a shared account. See Using a single iCloud account for Find My iPhone for more information.
Further, while you could restore an erased iPhone from an iCloud backup—assuming you’re backing up your iPhone to iCloud in the first place—anybody with your iCloud Apple ID and password can also easily erase your old iCloud backups as well. This would normally have to be done deliberately, but again anybody with your iCloud password can basically do anything in your iCloud account that you can.
If this is a concern for you, then it’s probably a good idea to not share an iCloud account—at least not for features such as Find My iPhone or iCloud backups. Subject to a few limitations, you can actually configure multiple iCloud accounts on a single iOS device, and it’s also important to keep in mind that your iCloud account does not have to be the same as the account you use to purchase apps and media from the iTunes Store. So you could easily continue sharing a single iTunes Store account to keep your purchases consolidated while using separate accounts for iCloud. See Sharing iTunes & iCloud Accounts in a Family and Separating family iTunes and iCloud accounts for more information.
Further, it’s generally a good idea to use separate iCloud accounts to backup each of your iPhones; since each iCloud account gets its own 5GB of free storage, you effectively get twice as much this way and don’t need to worry about one of you using up all of the backup space should your backups grow too large.
Keep in mind that on a given iOS device, the Photo Stream, Documents & Data, and Storage & Backup features can only be enabled on the primary iCloud account, and Find My iPhone and Bookmarks can only be enabled on one iCloud account, although it doesn’t necessarily need to be the primary one. On a Mac, however, the Bookmarks and Find My Mac features can only be enabled on the primary iCloud account.
Other iCloud features such as Mail, Contacts, Calendars, Reminders, and Notes can be enabled on multiple iCloud accounts at the same time, and this can be a handy way to maintain a shared address book, since unlike Calendars, iCloud doesn’t provide any way to share contact groups between multiple users.
In the very least, you can easily create a second iCloud Apple ID and simply enable the Find My iPhone feature there to isolate it from the shared iCloud account, which would limit the ability for anybody else sharing that account to remotely erase your iPhone. As noted above, however, there may be other reasons to separate your iCloud accounts, so it’s worth looking at what information you’re sharing and deciding how to best proceed from there.