My friend had a stillbirth, what tests are normally done to find out why?

Autopsy. The main test on the baby is an autopsy to look for birth defects or other causes for the death. The placenta is examined and cultures done. The mother may have tests for viruses and other infections, diabetes, thyroid or bleeding problems. Even with all these tests it's not unusual to not find an answer.
See below. If indicated the fetus and placenta can be sent for genetic testing to see if something untoward occurred.