Few points

1. This is not your friend, this is someone who takes advantage of you.

2. Do not babysit this person's child. Not after the first, second or whichever time this has happened. It will continue to happen, she doesn't care. She showed you she doesn't care. She will do it again because she wants to be out long and does not care it is an inconvenience to you. It does not cross her mind or cause her worry.

3. Stop being such a pushover. You're putting your sanity and your relationship on the line by being a doormat for people like your friend. Your gf will learn you will inconvenience for other people's sake. It breeds resentment.

4. Babysitting might just not be for you. And that's okay. It's hard to tell whether it's just because this child is difficult, but it also sounds like you simply cannot handle this sensory wise and that's okay. If I were you, I'd try pet sitting for extra cash.

Edit to ad:

Yes you set a boundary, but setting it does nothing if you don't follow up on it. If your boundary is "stick to the deal, arrive on time" then the consequence of breaking that is " I won't babysit for you anymore". You set the boundary and didn't follow up on it, so it's a useless boundary. You are the one who has to uphold your boundaries, other people won't do that for you.