What version Bible should I buy for a Seven Day Adventist?

Adventists often choose to use any from a range of Bibles.
The standard that everyone would refer back to is the King James version (KJV).
For modern adventists though, they would probably prefer to use something with more up to date language but with a good translation. Ones worth looking at particularly are the New King James version (NKJV) or the New International version (NIV). Stay away from any Bible that states it's a paraphrased version or the Douay-Rheims version (That strongly favours Catholic beliefs, which would offend Adventists).

Note: Seventh Day Adventists are not known as Mormons. That would be the Church of Jesus Christ of the Latter Day Saints. Giving the book of Mormon to an Adventist would get you some very odd looks (they might think you are a Mormon and trying to convert them).

A Bible would be a very nice gift to give an Adventist, but do be careful which version you choose (KJV, NKJV & NIV are the best choices). The nicest editions of any bible will have ribbon bookmark and used beautiful red text for any words spoken by Jesus.

If your Adventist has a sense of humour, you might want to also check out the Australian slang version of the Bible. This is definitely very tongue in cheek (and not for any serious bible study) and very funny to read. Yes it is a proper translation of the New Testament only into Australian Slang. You can buy a copy in Australia from Koorong Books.