I do love a good CF thread!
I can never understand people who think they'll be seen as a bitch, or worry that they'll get some head biting comment back. The CF is relying on you feeling too awkward to stop her getting away with it, that's her only strategy. You don't have to give a damn whether she thinks you're a bitch or what her reaction is, all you have to do is be able to live with yourself.
Personally, when somebody has been rude to me, I don't consider it a problem to stop being polite to them. Expecting you to pay for her repeatedly without ever paying you back is rude in the extreme.
Things that I would have no hesitation in replying to her;
Buy me a sandwich
No.
No and please don't ask me again.
Kinda awkward of you to ask, Karen, you haven't paid me back for the last few!
that's the third time you've asked me to buy you food and you've never paid me back. Doesn't it get awkward? [allow vacuous silence]
I've forgotten my purse
..... again? [disbelieving face]
That must suck.
But how will you pay? 
But it's your turn to pay
That's weird, I mean it's pretty hard to forget something all women are in the habit of carrying in their handbags with their keys and other essentials
LOL I had a friend who kept "forgetting" their purse. Turned out they thought other people would just buy then stuff and they'd never pay it back. What a CF!! Glad I.Don't fall for that any more
So go back and get it
Can't you just sub me
No, because you never pay me back.
I'm not an ATM.
Buy the BOGOF drink and I'll get the next one
But you always ask for that and never buy the next one 
Great idea, I'll have the pornstar martini, it's your turn to get them in, while you get served at the bar I'm just going to the loo.
That's a great idea, let's all get two drinks each, what are you going to have?
Mmm no, I'm fancying this other drink.
Hmm no I'm fancying this other drink. Hey you get them in, I hate waiting at the bar, and I'll go find us a table, thanks love!
But you said that last time, then we all sat there with empty glasses and you didn't get another round in 
No thanks, you never reciprocated or paid me back last time
LOL yeah nice try its your round, cough up!
I'm collecting these coins
[Loud laughter] Oh honestly Karen you're so funny! 😂😂😂😂 [silly voice] "I'm collecting these coins" 😂😂😂 I must remember that one it's a hoot!!^
LOL well don't look at me, I collect ALL of my coins and notes!
I'm serious, I want you to buy me a sandwich because I can't spend the coins I'm collecting
No love, I'm not going to buy you a sandwich when you're standing in a sandwich shop with your own money. That would be ridiculous.
Let me get this clear, you just want to spend my money not yours? Not happening, and I think it's rude that you would ask or expect that.
^So pay on card, go get more cash out or