Don't let her take your son man, she sounds reckless. I'm sorry you're havin trouble with love. there are other women out there, don't worry.
If I were you I'd stop smoking around her, if she really wanted to fuck you over if it came down to custody I guarantee you that shit will blow up in your face.
Hate to say it, but if you love your kid you'll fuck her over before she fucks you over. You think she won't? she's a woman. The legal system fucking caters to woman. Protect your shit. Seek legal advice now.
My mother had me at the same age as your girl had your son and she acted the same way during my childhood. I love my mother, don't get me wrong, but most of my childhood memories are of her going out and partying all the time. I remember bawling my eyes out several nights a week because my mom never wanted to stay home with me and my sisters. She'd stay out all night, wouldn't come home til the next day or 2 after. It hurt me so much that she'd rather spend her free time partying than being with her children. I know she loved me but it was hard for me to feel. Her actions definitely spoke louder than her words. Now that me and my older sister moved out, she regrets wasting so much time on "friends" that don't even know her anymore. Like I said, I love my Mom but we aren't close at all. I don't feel a strong sense of attachment towards her because I eventually learned that wanting to be with her only made me hurt more. So, if you get a chance let your (ex)girl know that her child will only grow up once and those are precious moments she's missing. Let her know that if she continues her habits, she shouldn't expect to have a meaningful relationship with her kid in the future. Sad but true. When the partying gets old and all of her friends are gone, who will she have left?