It's pretty presumptuous to make that assertion unless you knew the person for many years in different work environments.
I've seen lots of engineers become 'bad engineers' because of a myriad of environmental issues (bad management, bad peers, bad office, bad engineering decisions foisted upon them).
If you didn't see them perform well, don't provide a reference. If they are that bad, nobody in a reputable position will provide them a reference and the hiring company will see that.
It's not your responsibility to be an arbiter of some ex-coworker's life.
The only time I was a reference for someone, I was asked questions, and I answered them. I wouldn't know if they were spun positively or negatively. Is that how senior engineers provide reference, or is it more free form?
If someone calls me up and asks about Coworker X, and I refuse to provide a reference, isn't that likely to be viewed the same as a negative reference?
I've seen lots of engineers become 'bad engineers' because of a myriad of environmental issues (bad management, bad peers, bad office, bad engineering decisions foisted upon them).
If you didn't see them perform well, don't provide a reference. If they are that bad, nobody in a reputable position will provide them a reference and the hiring company will see that.
It's not your responsibility to be an arbiter of some ex-coworker's life.