Keep in mind that NO form of birth control/pregnancy prevention is 110% "fool-proof". Condoms can fail as can birth control. A lot of times, it's a result of user error. With condoms, using one that's too big or too small can cause failure, as can using an expired one or re-using them. With birth control, it's important to take your pill the same time each day (it's ok if you're like 5 minutes late one time or something). It's also important NOT to skip any doses. If your girl would forget to take one, she should take it as soon as she remembers. If she doesn't remember until the next day, she should take the pill she missed AND the pill for that day. Usually, you're ok unless you've skipped 2 or more doses in a row. If you've done that, you need to use condoms in addition to birth control as a backup for about a week until you're back on track.
if your girl has any questions, she should speak with her doctor or the pharmacist. They can help her determine which type of birth control will be best for her. I use birth control pills and my guy ejaculates inside me quite often. No babies yet because I take my pill on time each night. As long as your girl does the same, then yes, you'll be able to ejaculate inside her without getting her pregnant. That's the purpose of birth control.