Can you be promoted to MSG/SGM with a substantiated IG complaint?

For MSG the board members look at whatever is in the promotion file. Whatever you see when you open your board file is what the board sees. If a person has negative information available to view on their board file, then they will be rated lower than their peers. The same is true for the board to promote to SGM. The evaluation board reviews your promotion file, which the Soldier reviews first.

However, those who are selected for SGM undergo a separate review, essentially a suitability review, looking at all available databases, to determine if the Soldier meets the character requirements to be a SGM. Will the Soldier pass that review? Who knows... You can't begin to guess what the policies will be 5-15 years from now, or even if the Soldier will ever be competitive enough to be selected for SGM in the first place.

But the promotion board does not access multiple databases while rating packets. The packets are pulled and compiled ahead of time for the board members to review

Immediately after the USASMA selection board and CSM/SGM promotion selection board recesses, the process will include a review of information contained on the restricted section of the AMHRR, substantiated DA Inspector General reports, reports filed with the U.S. Army Crime Records Center and the Central Clearance Facility.
All information received from the agency checks is forwarded to the Director of Military Personnel Policy, for a U.S. Army Senior Enlisted Review Board (USASERB). The USASERB reviews all alleged derogatory information and determines if the information warrants referral to an Enlisted Standby Advisory Board (STAB).

Your friend will be notified and given the opportunity to submit a rebuttal in her case for the STAB.

https://www.hrc.army.mil/content/Post-Board%20Suitability%20Screening

However, if there is paperwork showing substantiating evidence, then whatever NJP comes and up to and including an NCOER reflecting a valid IG complaint, then one can guess that a SM would more than likely not be selected for MSG/SGM