How to export a Sonar x1 project and be able to open it in Cubase ?

You must find out if Cubase supports OMF files. If it does, you must save your project as OMF. That's the only inter-DAW project format. The other option is to save each track separately as a Broadcast Wav, and your friend has to import it track by track into a new project (preferably to one with tempo originally set to the exact tempo of the project).

Someone savvy will surely chime in to tell more details, as I have never used OMF. I do think, though, that your project must be all audio without VSTs even when using OMF..

Remembering some threads on this subject I don't think even OMF guarantees that there will be no problems.

When working on collabs, the only one who really needs WAVES is the person planning to do the final mix. All others can do their supporting work easily by using the MP3 versions and importing them into their DAW's.

It's 'having' and 'using'. Not 'haveing' and 'useing'... Sorry, but that was really bugging me... If it's for a remix like you say, prob easier to go with .wav stems. You can probably get away with mixing all drum tracks down to a single stereo file. Group other similar things too, like all vocals together, or main vocals the all harmonies grouped. Stretch the lead in time of every stem back to the start then export the .wavs. When they are imported into cubase, they will thus line up perfectly. Talk with the guys and find out what separate instruments the might want control over to minimize the number of stems you need to make.