Well, the problem is that once headers have been sent due to output, e.g., because of an echo/print, then using the [man]header[/man] function, setting a cookie with [man]setcookie[/man], or starting a session that sets a cookie will fail with the error that you encountered. So, somehow you must structure the code such that the output is not made until the right moment, or perhaps more lazily use output buffering.
I suggest that you contact the plugin developer to ask for help, perhaps reporting it as a bug.