Oh man that is a hard one. When my H left he needed to update our tags on our cars. Mine was fine but he was expired. This was before I joined the website. I sent a friendly hey don't forgot your tag expires on your birthday. He called me a nag and said he can handle it and doesn't need me always telling him what to do. Guess who got a ticket a week after for NOT registering the tag. Mhmm. Since then I don't say anything.
Is his mail, drivers license, etc still with your address on it? Like could it be sent to your house or use that address and later on he can switch it? If so I wouldn't say anything. If it is a one time thing where you HAVE to have it updated and changed right NOW then I probably would let him know but that is just me.
