Hard conversations with customer? When your boss doesn’t have balls

people in strategic account mgmt - you might have had situations where you just need to have that hard conversation with customer about how they are getting in the way of their own success.   

..... and you have a boss (recently came over) who doesn't know how to say "No" or always been known to give-in what ever a customer asked. Gave me long lecture on how every customer needs to "like" me for me to be successful in my role.

How do you deal with this situation? how do you get your boss to support you when they don't have balls to see bigger picture? (doesn't make it easy to have difficult conversations with herself)

any tips ? past experiences ? please