Mr. 1066, math and roleplaying are independent variables and inability in one is not indicative of inability in another. I can't reliable add two-digit numbers and I've been roleplaying up a storm since 1979. The math portions of my brain have largely been co-opted for story and character skills, which are much more useful to me as I have a husband, a calculator, and in extreme cases a hired accountant for the numbers, but have to write my own stories, or they won't get written, much less published.
Numbers are arbitrary symbols which convey little meaning. That's what they are to me in real life, and guess what? That's exactly what they are in the game! They exist to represent relative values. That's it.
If you have someone with actual math anxiety, take care to assure them that the numbers aren't actually math here. They could be colors or letters in a different system. Yeah, you have to add and subtract modifiers, but those can be counted on the fingers if necessary and will always be written down for handy reference.
It used to be SOP, back in the day, to start newbies off with fighters because they were simplest. I'm not sure that's true anymore. Ask her in very general terms what kind of hero she'd like to be and point her to the classes that suit that best. Most likely she won't have a class in mind. Classes are as arbitrary as numbers, but they're based on images and stories, so let her pick the class first, then roll the dice, then explain to her which abilities should get the highest numbers in order to be most effective in that class. She'll get that. Then walk her through the rest of character creation, making her write things down by hand - that way stuff will be arranged in a way that's intuitive for the player, not some random person designing sheets at a game company. Be ready to say things like: "yeah, it can be confusing, but don't worry - everybody forgets to use something once in awhile."
In play, the player tells the DM what she's doing, and the DM tells her which die to roll, and whether she wants low or high. Seat her next to somebody patient who can keep an eye on the modifiers, reminder her which ones are relevant, and won't yell at her if she gets confused and adds her ability score to a die roll instead of an ability modifier. If she gets that deer-in-the-headlights look, ask direct questions: "Forget what you can do right now, what does your character *want* to do?" "Um - run away. Terribly fast." "Okay, if you do that you'll have to run past three of the orcs, and each one gets a chance to hit you. Do you want to risk it?" Let other people advise her freely without regard for normal restrictions on what PCs can talk to each other, unless they inundate her with conflicting advice.
If she has the game in her at all, after a session or two of this she'll settle down and find style of play that doesn't slow you down too badly. Some people never do get the hang of the dice, but an awful lot of things in the game don't need the dice.