No, he will not get booted downstairs when he drops to blue, but he will no longer be able to move his seat if a better one opens up.
I agree that there's no harm asking if he's a long-term silver 20 points short of renewal, but wouldn't bother otherwise.
I really don't think that the ash cloud is worth mentioning; we were stuck in the Maldives and were given a confirrmed flight about four weeks later even though there were odd seats cropping up earlier, which wouldn't have helped much to clear the backlog given more than three full flights of PAX stranded (I did book a couple of fully refundable CW singles for two weeks later just in case I needed them). BA moved us to another resort and paid everything for the week we were stranded (well, only 50% of our bar bill
), which was more than could be said of other non-EU airlines and charters. In our case it was good PR BA.