Deceased author and conflict of interest statement
There won't be any standard way for this to be handled, and it's not really up to you, directly. You'll need to discuss this with the editor; I'm sure they will work with you to make this not an issue, perhaps either by having the other coauthors speak for the deceased or simply waiving the requirement.
How to acknowledge a deceased advisor’s contributions to a paper? and Dead author ethical guidelines may also have some relevant content for you as far as authorship for deceased contributors - since the paper was so near to submission I think it's straightforward to argue that your colleague would have approved submission and that they should be an author, but more generally the journal should be made aware that one of the authors is deceased, since they may have relevant policies.