Readline is distributed only under GPLv3 nowadays. I worked on some software licensed only under GPLv2, where some authors are no longer available. GPLv2 and GPLv3 aren't compatible.
He probably wanted to link the Readline library into his GPL2 program, which according to the owner of the library (the FSF) is not permitted and might have led to his employer's getting sued by the FSF for copyright infringement or at least to his employer's lawyers getting scared, then badmouthing him to management.