ANSWER:

Your friend is correct. Witr is the last prayer you pray for the night and should be prayed after tahajjud.
But if you are afraid not to wake up, you can pray it before going to bed as you are doing at the moment. 

I usually pray witr after isha prayers and wake up around 3am and pray tahajjud and later fajr. But a friend said that witr should be prayed after tahajjud since it should be the last prayer of the day. What’s the correct opinion on this? Im confused because i thought tahajjud was supposed to be considered the next day