Does my girlfriend need Thai insurance for her long stay visa, and how should she handle her flight itinerary?

Your girlfriend does not need to use a Thai insurance company specifically; the insurance policy must simply provide coverage for medical expenses, including COVID-19, for at least $100,000. As for her flight ticket, it does not necessarily have to show a return flight date exactly 60 days from the time of application, but it should reflect her intended duration of stay.