As a PL, what do I do if one of my soldiers is arrested for domestic violence?

First step will sound like common sense, but the first step is always to inform your company commander immediately with the 5 Ws, Who the Soldier is, What he is charged with, Where it happened, When it happened, Was the arresting officer police or MP, the report number and POC for the report, does the Soldier have personally owned weapons, and where the Soldier is right now.
On active duty, the normal process for a domestic is to rescind the Soldier's pass privilege of living in post housing or off post and move him into the barracks to remove the threat.
The Soldier will need to be flagged immediately for Law Enforcement Investigation, and will probably need a Behavioral Health assessment with escort scheduled.
Your Commander will need advice from JAG, your Soldier will need advice from TDS.
If the Soldier is convicted of a misdemeanor domestic violence crime, it now becomes a felony for the Soldier to possess a firearm or ammunition, that includes an M4 from the arms room, which means no qual range, no deployment and bar to reenlistment.

IG already mentioned is a good start or the SJA as well. From a leader perspective, you can help ensure that both the Soldier and spouse are “safe”. In that I mean, where is the Soldier staying (assuming they cannot stay at the residence) barracks, fellow soldier, etc. Ensure the spouse has access to funds, especially if there are children involved. The Chaplain, MFLC and other services are also there to help the Soldier and spouse going forward.

Counsel them on the implication of a conviction. Beyond that, I would play wait and see if they are convicted and for what. I have seen plea bargains where they pled guilty to something else as opposed to domestic violence. In that case, you don't have any Lautenberg amendment issues (assuming it isn't a felony). You could go the UCMJ route with them, but I have always been opposed to doing that if the civilian courts handled it. Others may have a different philosophy so opinions may vary on that one lol.