"Look, I'm going to be honest with you: your pentesters are morons. They're grossly incompetent and should be embarrassed. I can give you a list of qualified alternatives you might want to choose from, and not just to test the work I've done for you, but for all your other projects too. Seriously, their advice is just awful and you really need to switch."

This isn't the time to tread lightly, but to go scorched earth. This isn't an "oh, we disagree on the finer points!" debate between peers kind of situation, but a flat-out "these knuckleheads are putting you at risk and you need to know it". You want to get the point across that you're not messing around or leaving room for doubt.

Source: have had these conversations several times over the years. I normally pride myself on tact, but in my experience tact is the exact wrong approach here as it gives the client the impression that there's a wiggle room of doubt.