I've tried searching for the answer in a lot of place. But all I get is formal answers (like 彼女はいつ来ますか). I'm assuming that the informal style will be 彼女はいつ来てるのか。 because the verb come turned into coming and is acting like a noun so, I added a の at the end of it. Is it correct? Google translate doesn't do a good job with informal styles. So can anybody say how a native Japanese a boy would say this to his high school friend? This will give me ideas to apply this to a lot of other places. Thank you so much in advance.
The informal version of "彼女はいつ来ますか？" is "彼女はいつ来るの？"
- "のか？" is a literary style, and it almost never happens in ordinary conversations in the real world.
BE + -ING(eg "She is coming tomorrow") is an English way of expressing future tense. Japanese ている doesn't have such a function.
(By the way, if you want to make it really natural, don't use 彼女. An average Japanese boy doesn't use it more than once a month.)
I'd say 彼女はいつ来るの? Just adding の right after the base form of verb works. This rule can be applied to other verbs like 戻る, 帰る, and so on.
However, Japanese don't use 彼女, 彼 or 私たち so often. I actually pictured myself speaking and figured out that it would be more natural if we omit subjectives or specify persons' names. e.g. いつ来るの?, Mary はいつ来るの?