Proper interactions are a problem for many kids on the autism spectrum and some parents (myself included) have found success with “social stories.” Social stories were developed by Carol Gray, a long-time teacher of students with ASD, to help them behave appropriately in various situations. I used them quite frequently with my son when he was younger and found them extremely beneficial. They'd be ideal for your 5-year-old granddaughter.
Type “social stories” into YouTube and you'll find many examples of animated social stories. They include common situations that kids encounter at school and at home such as “Keep Your Hands to Yourself,” “Stay Seated on a Bus,” “Always Be a Good Sport,” “Being Teased at School,” and “Put Your Toys Away.” They're short and the messages are conveyed in a cute and engaging way that's clear and concise. I'd watch them with my son and then we'd practice the appropriate behavior step-by-step. When he got it down, he'd earn a sticker on his chart.
In a unique situation like your granddaughter's, it's recommended that you create your own social story with words and pictures. However, to do so, you would need insight into why your granddaughter did what she did. Was she trying to be funny? Was she being bullied? Was she trying to become friends with the girl? A social story isn't effective unless you can pinpoint what caused the behavior and, therefore, can find the remedy for it. A social story about how to handle a bully would be very different than one about how to make friends with someone.
The psychologist or resource teacher at your granddaughter's school probably has a collection of social stories in her office. She would also be able to help you write one about the milk incident (they're created in a very deliberate way and have certain essential elements). If she's not currently working with your granddaughter one-on-one or in a small group, you should request that she start. You shouldn't be doing this alone but with a professional.
Additionally, if your granddaughter is struggling with peers because of speech issues (articulation, social communication, receptive or expressive language), she should be working with the school's speech therapist.