Can I sue someone for recommending a professional who turned out to be negligent in Ontario?

Generally, no - simply recommending a professional, without more, doesn't make the person who gave the referral liable for that professional's later negligence. Ontario law doesn't treat an ordinary referral, whether from a friend, another professional, or a business contact, as a guarantee of quality or as creating a duty of care over how the recommended professional actually performs. The negligence claim belongs against the professional who did the work, not the person who suggested them.

There are narrower situations where a referral itself could create exposure: if the person making the referral held themselves out as vetting or guaranteeing the professional's competence, made a false or reckless representation about their qualifications or track record, or had actual knowledge the professional was unqualified or previously negligent and stayed silent about it. A professional who refers work to another within a formal referral or fee-sharing arrangement can also sometimes carry more responsibility than a casual personal recommendation would. Outside those situations, the practical focus should stay on the professional who actually performed the negligent work, since that's where the real duty and the real claim generally live.