2×6 rafters “might’ be able to span 10′ but only if the snow or wind loads are less than 50psf.
A girder made from 2x6s can only span a maximum of about 6′. No way will a 2×6 girder be able to span 16′ even with a steel flitch plate. (Girders made with flitch plates must be designed by an engineer and approved by the building code official.) Flitch plates are purchased at steel suppliers.
You’ll need at least 3 support columns for this girder no matter the design. Even a girder made from 4 2x12s can’t span this distance. Only other alternative may be to have a steel girder, glulam or other engineered designe, but for a 16′ span will be excessively deep.
The roof will also not meet code. The roof needs to be designed and installed just like any other roof. Unless she can demonstrate to the code officials that the PVC panels can support up to 40psf, it will not be allowed.