Anything. She needs to say literally anything because right now, this is the beginning. You don't have to start out running. You can warm up first, stretch, and speed up at your own pace.
Your friend's sister, 16, was silent and afraid chances are it's because she froze. That's okay. He wanted her to be afraid. That was what he was going for. To have a fight/flight/freeze fear response; that behavior is normal.
I have no idea if she has any experience with any people of any gender or size disrespecting her in and taking what was she had claimed as hers in public but based on your description I would guess it hasn't happened often. The first shock of realizing that some people really are going to treat you like that, it's pretty awful and it can freeze you up. I know for me the only cure was practice, and I was well into my 20s before I could look a someone in the eye and tell him to get back in line without tripping up and I still choke sometimes and I know a lot of people who do.
If she's freezing up rather than stuck on what to say, she should start, if she wants to find a way through, by saying anything. It doesn't matter what. The people above in this group have given great suggestions of what to say to be heard and what how to present yourself with authority but I want to address something else - what it's like if (or when) a person aren't instantly able to stand up for themselves.
As long as she opens her mouth and get a sound out, even if all that comes out is a squeak or a "hey", it's a win for the situation. It's progress. It's exposure to the stimulus and building experience in confrontation which is incredibly hard and is going to trigger that freeze response, made more difficult if standing up to someone who is trying to intimidate. Since the response is going to be to freeze most often in that particular confrontation which is why the push through to anything is so important.
What people don't talk about enough is that no matter what comes out - even if it's a flawless remark that is composed, confident, and witty - she need to be prepared to be ignored, told no, be insulted or otherwise have a negative outcome. That possibility is very very real and when it happens, she should brace herself for physical reactions like dry mouth, blushing or flushing, stammering, sweating. The emotions it can cause can be really powerful too - usually they're embarrassment, shame, weakness, a sense of being small, and defeat. It's going to happen repeatedly. Probably often.
I don't say all that awful stuff discourage her. On the contrary it's because standing up to people is a skill just like everything else and well prepared is well armed and well informed is well prepared. If you know those symptoms are coming then you know you'll live. Those feelings aren't forever. They fade quickly especially when focusing on the fact that even small practice is forward progress and more than the time before. Faced, it is something that can lose it's fearfulness.
The thing I've noticed in myself and in my friends is that its not what to say that's hard. It is the confrontation itself. You know that you want to say something but since we know how it feels to fail a confrontation but most people don't practice it and inure themselves and inuring the feelings of failed confrontations - when you need to say or do something, the action won't come over the fear of the consequence. It's why that guy's tiny awesome girlfriend in the other answer is able to do what she does. People freeze when faced with confrontation because they don't have the practice, she does. The serving the first volley like Mr. Big did is easy. If you have enough practice under your belt that you can actually come back with the return, you've usually won. It is rare for it to go back to the server.
So, all the things here about acting and presentation and what to say, please, do take them back to her. But also take this if you can. Practice actually confronting small personal injustices with something, anything. The more she does it, the more power she'll have to use when she really needs it no matter what it is that she wants to put it behind.