What's a good amount to give as a graduation gift for this girl?

My friend’s daughter is graduating from high school. The mom (my friend) used to work in the same department as I did, so we’d see each other at work, to chat and shoot the breeze. They’re in the same school district as we are, and the brother is in my daughter’s grade.

I’m not close friends with the mom, meaning we don’t do things together except see each other at school events and the annual holiday party here. We don’t talk on the phone, we text. I don’t know the graduate at all, other than being her mom’s friend.

The party is going to be one with a DJ, so it’s a bit more than just sandwiches at their house or something small. My teenage daughter is invited, too, but I am not sure if I am taking her as she wouldn’t have fun.

What’s a good amount to give as a gift? I’m thinking $50 but I am totally open to other ideas, including giving an actual gift instead of money.