Yo soy su amigo --> I am her friend?

In lesson 1.11, without more context, I'm not seeing how to know “Yo soy su amigo” translates to “I am her friend.” Did I miss something?

Without context, you are correct. It could also be “I am his friend.” and “I am your friend." (formal)

I find myself often saying “I am his/her/your friend." to acknowledge it could be any of them, unless one of the clues (fem for example) is present.

Thanks! I appreciate your help.