Wow, the guy unknowingly caused you to lose your friend but it wasn't intentional, and since you had not the intention of what your friend thought then you don't owe her an apology.
She was childish and I understand why, you should have understood that you can't force people to love you. The guy wanted you but you wanted her.
So what happened to the guy's proposal after? Did you by any chance become friends even if you didn't date