Flying Blue give 50% mileage on R and T fares on China Eastern. LHR-PVG-MEL is about 10,700 miles one way, so at 50% she'd earn 10,700 per return.

A classic award LHR-MEL with Flying Blue is 100,000 miles, so ten returns would earn her one free trip. Flying Blue miles remain valid indefinitely, as long as she makes at least one flight every 20 months.

I don't know enough about other SkyTeam FFPs to be able to make any comparisons. However, if all else is equal, it is generally worth using the FFP of the airline you fly with most often (as airlines may give slightly better benefits to their own frequent flyers, than to those with other airlines in the alliance). So, she should certainly check the details of Eastern Miles as well.