You're in luck - there are a number of companies (such as IntellilinkMods.com) which can fully fit a reversing camera and programme it into the existing screen for as little as £120. We'd recommend going in that direction rather than fitting rear view mirror cameras.

30mpg isn't particularly bad in town driving and if traffic is busy. It's worth considering her driving style to increase MPG: anticipating what's ahead, less hard acceleration, more coasting off throttle and less hard braking will help.