It needs to be: I know a guy who is looking for a job.
So, the full sentence needs to be: I know a guy, whose mother is a veggie seller, who is looking for a job.
It seems rather an odd sentence - why should the fact that his mother sells vegetables be of interest when we are talking about a man who is looking for a job?
'Veggie seller' is not a term that I think many people would use? Did you consider 'greengrocer'?