Ok, sorry, I read that as you didn't want to say anything to the joint owner because they weren't related. My bad
Is this an elderly person? If I were concerned, I would talk to the joint owner (without implying that you think the customer is mentally deteriorating, they could take offense to that). Simply a phonecall to say that you really value your customer, etc, but that the branch (or whoever) was recently concerned about some transactions or conversations (whatever the situation may be) and that you want to ensure everything is ok and that nobody is being taken advantage of, etc.