I think given the seriousness of his health, I'd park this issue for now in your mind, wait until he's home and settled into a routine before addressing it.
In the meantime however, I'd be going through his statements with a view to finding out:
What he's spending on and how often
Is he making the monthly payments on time
Is he paying the monthly minimum only? (if done over a long time this means the total may continue building even when you're not making new purchases)
It's possible that he used the card initially to fund a large purchase - car, large appliance/electronics, holiday, housing repairs/work - with the intention of spreading the payments, then has ended up frittering away a few quid here and there and then it's built up to a worryingly large amount, and he's carried on doing the same thing and burying his head in the sand. It's very common.
When you come to discuss it, explain exactly how you found out, and then just listen to what he says. Then decide as a team how to approach paying it off, and that will probably go hand in hand with a review of all your finances. If you have savings, it will almost certainly be cheaper to pay off the whole outstanding amount with those, then work on building the savings back up.
Money saving expert website has lots of advice about reviewing your spending and setting clear financial goals going forward.