If someone funds another person's will challenge, can they be ordered to pay costs in Ontario?

Yes, in appropriate circumstances. Ontario courts have the power to order costs against a non-party who funded and had significant control over litigation, including a will challenge, where that funder effectively drove the litigation for their own benefit rather than merely offering incidental support to someone with a legitimate claim. This is a recognized, if relatively uncommon, exception to the usual rule that costs orders are made only against the actual parties to the case.

Whether a court will make this kind of order depends heavily on the facts: how much control the funder had over the decisions in the litigation, whether they stood to benefit financially from the outcome, whether the funded party could otherwise satisfy a costs award on their own, and whether it would be unfair to leave the estate unable to recover costs from a funder who was, in substance, running the case. A person who simply helps a family member pay a lawyer out of goodwill, without directing the litigation or standing to gain from it, is much less likely to face this kind of order than someone who has effectively taken over the case for their own interest. Get legal advice before funding someone else's will challenge, since your own exposure can be real.