I think I’d feel worse about sabotaging her chances of getting a job than I would about neglecting to tell her about a job.
Plus, @janbb might be very assertive and quite content to seek out the people doing the hiring and providing an alternative view to a recommendation but I think doing so could reflect negatively on her own reputation. As the person hiring, while I might appreciate the additional advice, I might see the person giving me two accounts as sneaky and lacking integrity.
The person might never ask for a recommendation/reference. @janbb would have more sense of whether that’s likely to happen. However, I work for a large organisation and unless it’s a few hours work, all applicants need to provide references. The process calls for applicants to formally list referees and HR sends out a form that has to be completed by the referee. If @janbb were listed as a referee, she’d then have to give a bad reference. There wouldn’t be an opportunity to go through the back door and sabotage the process.
Plus, sometimes people come to me and ask for a written reference they can attach to their application in addition to their formal referees. If I don’t rate the person, I’m put in a situation where I either have to refuse their request or give a mediocre reference. Either can be awkward, but it’s better than a good reference circulating with my name on for a person I don’t rate.
Hence, if I don’t want to work with someone, I just wouldn’t tell them about an upcoming job. If they find it themselves, so be it but I don’t need the awkwardness of dealing with requests for references etc. if I can avoid it.