I was going to reply to your previous thread but its been locked for some reason.
The XER format is only compatible with the P6 Client (the windows desktop client, P6 Professional).
Both P6 Web and P6 Client support XML files so you should be able to use this to exchange the schedule with the other person.
It's worth noting that 8.3 introduced a host of new features around importing/exporting XML that allow you to manipulate some of the data as it goes in.
XML is definitely the format to start using going forward - that is where Oracle will be making the changes/improvements when it comes to import/export.