How do I approach a business owner about an unresponsive lead developer without sounding like I'm trying to undermine them? I WILL NOT PROMOTE

I recently started my own software development business, and I've been fortunate enough to land a fairly large client. The project itself has been difficult, not because of the work, but because the current lead developer (who has been working with the client before me) has been extremely unresponsive. Deadlines keep getting pushed back, emails go unanswered, phone calls aren't returned, and messages are often ignored. I have a paper trail of all of this if needed, but I don't want this to become a blame ga