Is it true that regular fruit juice contains alcohol?

Fresh fruit juice, no. Juice that has been allowed to sit around and ferment, yes.

Yeah, your friend is definitely wrong.
As @zenvelo says, only if it’s fermented.

Actually you can say you both are right. Here is a paper by Perkin Elmer that measures the concentration to incredibly small resolution.
Fruit Juice Sample Concentration of ethanol (mg/dl)
Orange juice A 56.5
Orange juice B 3.7
Mixed berry juice 57.0
Grape juice 236.2

100 ng/dl is roughly 0.000001% (Someone please check my math.)

I don’t know what you mean by “ng / dl” @LuckyGuy. (I’m assuming it’s just a typo, because the paper you link clearly shows mg / dl.)

But 56 mg / dl converts to 0.56 g / liter (link), and since there are 1000 g / liter (making each gram equivalent to 0.1% of a liter), then 0.56 g would represent 0.056% concentration.

@CWOTUS. Thanks!!! I misread it. You are correct. It is mg not ng. So 100 mg /dl is 0.1 %. Still pretty darned small.

It contains the possibility of alcohol.

Sure, if you let it sit around long enough.

Thanks, this was very helpful.

My friend was wrong.~