What do you think about me telling a high school acquaintance that he shouldn't apply to work for the same lab I work for?
Why don't you want your acquaintance to work at the same lab as you?
He is not fit or smart enough for this employer. I'm not sure I want him to be my colleague
I understand that you don't think your acquaintance is competent for the job. It's fine if you want to let him know how challenging your work is so that he understands the demands of the job.
It means that I have to help him realize that he sucks.
Believing that someone sucks based on what you know about them is not okay. You should give other people the opportunity to improve and enhance their skills.