Elements does not work with PR files. Continue using PR.
Welcome to the forum.
In some cases, PrPro can Open/Import PREL (Project files from PrE), but PrE has never been able to Open (Importing a Project is not possible in PrE) a PRPROJ file, just as Harm states.
One workaround would be to Export the Timeline from PrPro, and then Import (Get Media/Add Media, depending on version) that output file into PrE for further editing.
- Edit the Timeline very loosely in PrPro, without any Transitions, and adequate Handles on each Clip, then Export that for Import into PrE. Then do any final editing.
- Edit the Timeline very tightly, exactly as you desire, essentially finishing the edit, then Export that for Import into PrE. Then do just very minor editing.
Depending on exactly what you are working with, you might want to also consider using a lossless CODEC, like Lagarith, or UT, for those intermediate files.
Workaround? He cannot open the files.