I'm going to ask my friend to pay for her meal
Why are you going to ask her to pay for her own meal?
I am tired of paying for her.
Asking your friend to pay for her meal would be okay if you did not already offer to pay. It's not okay to back out at the last minute. Doing this might leave your friend with a financial burden.
What do you mean by financial burden? I'll pay for my plate and then ask her to pay for hers.
If you already invited her out and said you would pay you need to pay for it. Next time you go out you tell her she needs to pay before you even make plans, just be honest that you can't afford it.