How to persuade someone not to have lunch with you?

I have a friend who works near the place where I work, at the shopping complex over the road. Whilst my lunch breaks are normally at the same time each week, those of the friend are super sporadic and they usually turn up at my work (in which there is an in store café, where we take our breaks together) expecting for me to go on break immediately. I have explained that because I can’t contact them during working hours by mobile, I normally have already had no choice but to go on break because of other coworker’s needs etc. Now it is especially so as at the moment we are hilariously understaffed, and my breaks are going to be interrupted every two minutes, and that will render any break time we have together redundant. Having explained this to my friend, they are threatening to take it personally, so y’all, after all that… got any excuses/cutting comebacks that won’t seem like I’m trying to avoid this person? Sorry, these are basically twelve year old problems, but that is what happens when you accidentally befriend crazies.