I'm not sure I follow. You're saying the previous employer doesn't want the laid-off employee to get a new job? I guess that would vary ex-post (eg depending on the reason for the firing, and the industry - a professional services firm will be keener to have a successful alumni network), but I'd imagine that ex-ante having the reputation of 'we care of people even if they don't work out here' or a promise of 'even if this startup crashes and burns we'll do our best to ensure you land on your feet' would make an employer attractive.