First, make sure she isn't missing some tiny detail in the manual (or she can search Toyota's web site) where the car is not supposed to lock under certain circumstances. There may be a switch somewhere that prevents all locking until it's flipped.
If that's not the case: She should check the warranty. The car is so new that it should be under warranty. She should not wait -- she needs to get to the dealership that sold her the car and nicely (at first, at least) DEMAND they fix it for nothing.
If they are jerks or claim it's not covered by the warranty, she can do what my friend did with a different major purchase: She turned up at the shop with addressed, stamped complaint letters in her hand addressed to the local Better Business Bureau and the state consumer commission. She had copies for the business. She told them that if they didn't make things right, now, free, she would be mailing those letters the minute she walked out the door. That got very prompt action in her case. Your sister could also say she plans to turn them in to Toyota as a poorly run franchise and is considering taking them to small claims court too, if she needs to get nasty with them. There are "lemon laws" in many states to prevent car dealers from selling "lemons" -- the laws might not apply to this fix but it'll get them nervous if she throws the term "lemon law" around and says she's investigating whether her lawyer should look into that.....All this is ONLY if they're bad about it, though!
But I'm betting they will fix it quickly, nicely and for free because they won't want an unhappy and very vocal customer. Still -- she needs to get in there right away and not wait any longer.