Let's take a look at what you've written. First up:
I realise she has a difficult job in raising a small baby by herself but what makes me worried is that she is frequently extremely argumentative, always angry, judgmental, rude, irritable, has emotions which can chop and change in seconds, always stressed out and all her ex boyfriends and many of her very few friends, get slated by her
For starters---where is the father of her child? Is he at all involved, for example, paying child support, having visitation? If so, unless he's really irresponsible, he should be petitioning for full custody.
You also say she is "always angry" --- why put up with that?
I am really at my wits end and all my friends are telling me to dump her as she is to much trouble. She admits she is very hard work and I have asked her to get some help and she agrees, but always wriggles out of it saying she is fine. I do have feelings for her and love her son, but constantly feel stressed and will have to leave her soon or I will go mad.
You say all your friends are telling you to "dump her"---and you haven't done this because...?
You ask her to get help, she agrees only to later weasel out of it. Again, you haven't dumped her because...?
I don't know if she is bipolar or not but it is obvious that she does have some mental health issues. Do you really want to have to live with someone who won't get help? Do you really want to be miserable?
What troubles me most is that she has a child. I'm very concerned about her child. That's why I ask about the child's father. Someone who is related to the child really needs to intervene. If there is no one related to the child who is competent enough to do so, then the appropriate agency needs to be notified. Child Protective Services in the US; Child Aid in Canada, for example.
So...I would say move on. However, before doing so, try to get someone to intervene for the child's sake. Good luck with everything.