What does it mean if someone is unstable?

[Anonymous]

The guy I was seeing,
my friend (her husband was friends with him since they were kids) said he is a very unstable guy. What does that mean?

[Guy13]

That he’s not stable or balanced or reliable? Ask her…

[Asker]

But she is pregnant and i dont want to bother her

[Guy13]

I don’t know. Whatever you think is best.

[Asker]

Ok thanks. I asked her anyway, and i will wait for her response.