If INS is informed....
...then it will immediately launch verification by the inquiry and adjudications unit. If there is any criminal offense behind the separation, both the primary and dependent are in soup. If, simply the marriage broke and INS is convinced of a fraud in the marriage, again both of them are in soup. If, on verification, INS is convinced that the marriage genuinely broke-up, it will issue a NTD to the secondary. Else, it can still keep the matter in abeyance, pending final issuance of a divorce decree by the court, when it can peremptorily deny the I-485. I think, this is what INS does from a very pragmatic point of view that would avoid other judicial scrutiny of the service.It can reject citing the technical reason behind a marriage -- a conjugal relationship between the spouses as man and wife is no more valid. Doing so, it will attract scrutiny upon itself by BIA and Circuit Courts. That is a problem. It can approve, but, an inter-departmental inquiry into the issue (such as an audit) will put the officer who adjudicated such a case in soup. So, from a rational standpoint, as well as from a pragmatic standpoint to avoid any kind of scrutiny in future, INS will try to glide between Scylla and Charybdis. And that means, put the matter in abeyance and drag the issue until the divorce is finalized at which time the petition can be denied safely and conveniently.