Some of what Seventh Day Adventists believe is biblical. The most important thing is the deity of Christ which they do hold to. However they have some pretty peculiar beliefs as well. For example some Adventists will say that their denomination is the only one that will survive in the end and is God's remnant. Many modern Adventists have toned down this rhetoric. The biggest controversy around Adventists is regarding Ellen White and how some held considered her writings to be inspired and equal to the Bible.

As far as what to tell your friend... I would encourage you to focus on asking the right questions when you talk.

Ask her: What do you believe about Ellen White? How do you feel about other Christian denominations? I think you will be surprised, there are many people who attend churches of all different types that have no idea what they believe or what their church believes. Start by asking her questions in a friendly non-judgmental way and before you know it she will be asking you questions too!