A European citizen wants to fly from Europe to Canada with a change of airplane in the US.
I already figured out that the the participation in ESTA is required (if the country is applicable).
Does she/he need a return flight, or is the transit flight to Canada enough?
As a rule, countries do not require return flights, they want to see onward flights. The point is that you can demonstrate that you're only coming to visit temporarily, and not planning to immigrate illegally. So the US will be fine with your onward ticket to Canada.
Canada itself does not require return tickets for tourists, but they are strongly recommended, and you may be asked to demonstrate proof of funds or a plausible plan for when and how you're going to leave.
