Well in fact you cant open the new version of project in old petrel version - the reason is maintenance you pay to $chlubmerger , the only way to open this files is to **** binary files located in *.pet folder that process called reverse engineering and in order to do this you need some skills in programming.
The easiest way just ask you friend to send you the project in ascii format if it is input data
And if it is the model ask him to export grid in RESCUE format
If you can list the data you want to use i can advise you needed export formats