If they are good friends:
* Sí, son buenos amigos. (Yes, they are good friends.)
* Sí, son muy buenos amigos. (Yes, they are very good friends.)
* Claro, son muy cercanos. (Of course, they are very close.)
If they are not good friends:
* No, no son buenos amigos. (No, they are not good friends.)
* No, no se llevan bien. (No, they don't get along.)
* No, solo son conocidos. (No, they are just acquaintances.)
If you don't know:
* No lo sé. (I don't know.)
* No estoy seguro/a. (I'm not sure.)
* No sé si son amigos. (I don't know if they are friends.)
To ask for more information:
* ¿Cómo se conocen? (How do they know each other?)
* ¿Cuánto tiempo llevan siendo amigos? (How long have they been friends?)
* ¿Tienen algo en común? (Do they have anything in common?)
Remember to adjust the answer to the specific situation and the level of formality required.