I would say " jangan ganggu dia!" For instance when you see someone is being bullied, and you want to tell the bullies to go away, you would say " jangan ganggu dia!" However, when your friend is feeling down that she/he needs some time alone, then their friends try to approach her/him, then you can tell them " biarkan dia sendiri"

I think its more sounds right if you just say, "Biarkan dia sendiri!" or "Tinggalkan dia sendiri!" for the sentence leave her alone... :)

Yes, you can, but you need to change the word form. You should omit the prefix "me" from "membiarkan". "biarkan dia!" works well, but to sound more natural, we usually say "biarkan saja dia!' instead.

Membiarkan dia/membiarkannya is same as "let him/her alone" and not leave. Leave is "meninggalkan" in bahasa Indonesia. If you open Big Dictionary of Bahasa Indonesia, or also called KBBI (Kamus Besar Bahasa Indonesia), you will find that "membiarkan" and "meninggalkan" are two different words Membiarkan = not prohibit; not ignore Meninggalkan = go from; avoid from Examples: Saya membiarkan gelas penuh dengan air (I let the glass full with water) Dia meninggalkan kota Balikpapan menuju kota Penajam (he leave Balikpapan to Penajam) Hopefully you're satisfied with my answer ^_^

Let him/her