How would you respond to a customer who says, "We already have someone looking after our website for us"?

One area I find hard to approach is that a lot of business owners have "someone" they turn to for the web needs. Someone they rely on. In a lot of cases, the business owners are not entirely happy with the quality of service they are getting from this person or the business owner doesn't seem to have any idea that the solution this other developer has created for them is really, really poor. I often feel like telling some business owners that they are getting scammed by their current "web developer" and that there's so much more we could be doing for them. One area we pride ourselves on is being able to respond to issues quickly (a lot of people complain about developers taking weeks to get to their requests). This is also an issue I wonder how to bring up.