It is of no concern to have this behavior. Self talk is very normal for even those without a mental or mood disorder.
Of course if someone does have a mood disorder this is a very common thing. I do it all the time and it helps me remember things and get back on track when I get off of it.
Yes, it is very normal for someone who is Bipolar to talk to themselves. I am Bipolar and I did this before I was properly medicated. I actually hallucinated, heard voices...Is this person on meds for their condition? and if so, are you sure they are being taken as prescribed??
Also, there was a time when I was about 6 months into my therapy, on my meds, and went through a depressive state...I was medicated but was having hallucinations and talking to myself. There is medication that a doctor can give if a person experiences this.
I talk to myself all of the time.