There is always the possibility of birth control failing- the shot is one of the more successful birth control methods, with a fail rate of only 1%. She will need to make sure she gets the injection on time, every time for it to be that effective. In the end, it doesn't matter how many times you have sex, that doesn't increase or decrease the likelihood of failure of the method- it can happen if you only have sex once.
It's really not likely this method will fail, but if you are overly concerned, you can also use condoms. In the end, the only way not to get pregnant or get someone pregnant is to not have sex- but the method of prevention you are using is known as a very reliable one.