How should I counsel a soldier involved in an accident driving a military vehicle on base, but the accident is still being investigated?

Just write what they failed to do. Anyone can get in an accident that's not their own fault. Identify what errors in judgment they made, or preparations they may have failed to make. If you can't identify anything they did wrong, then refer it back to the person telling you to write it. Tell them to write the counseling.

Ok, so the got into a car accident. It happens. Start by verifying that the troop has a valid drivers license and has the necessary training to get a military drivers license. Next get a copy of the accident report. Or the officers report who was on scene. Then ask the troop to recount what happened, word for word. Based on this you should be able to get a fairly clear idea of what happened.

Based on the above, then sit down with the troop and have a loosely written counseling, you will later go back and type it up, and lay out that driving a privilege, and a serious responsibility and that involvement of a piece of government property in that is a serious event and should command decide, there could be UCMJ action involved. Lay out any errors that the troop made, if any but also lay out their positives that they made too.

Stress safety in your counseling. Let your counseling state the facts and observations and follow the regulations.

Lastly, once you have it typed up, I would avail myself of my CoC open door policy and speak to the 1SGT unless he was the one that directed you to counsel the troop. Show the counseling to the 1SGT and get his or her opinion on the matter. Then providing they are ok, present said counseling to the troop, and have them sign, date and add any remarks. Done.

This allows you to be thorough and neutral without railroading a troop and keep your CoC in the loop and for input. It may be a case of a NCO getting over zealous in recommendation for UCMJ.

Simple answer is you don't. Wait until the investigation is over. One of the most annoying things we see at the MP station is unit's jumping ahead such as getting an IO when the investigation is still open by the MPs. Let the MPs do their job, close the case, then take it from there.