Occam's Razor applies. Micky D's and other minimum wage jobs are often filled by college-age Americans. I worked at one beginning at age 16 all the way through college graduation.
Sure, we can't know for sure, but, again, why would she lie about something so minor? It's not like the idiots on JPP claiming they have PhDs, are medical doctors and the like.
She's middle-class, that's a fact. Where do middle-class college kids work? Usually minimum wage jobs. I have no doubt she had one. Wherever it was, she'd be claiming that. So if it wasn't McDonald's then where was it? The movie theater? A drug store? Another fast food restaurant? Why lie about working at McDonald's if she had another minimum wage job in college?
IMO, logic favors her telling the truth.