It is not a good idea to file them separatey.
Don't be in a rush to file your papers ... get a good lawyer to review his papers ... you say 'they are still assessing' ... please confirm that it is a lawyer that is making this assessment and not a non-attorney in HR.
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Yes, anytime before you are granted adjustment he can file. Once you are granted, he should also be able to file as a "following to join" until you become a U.S. citizen.
The above is intended only as general information, and does not constitute legal advice. You must speak with an attorney to discuss your individual case.
If you qualify for 245i - he qualifies.
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