Hmm ... that sounds so familiar, hating confrontation and worrying about a backlash! Doesn't make it right though, if one household unduly suffers while another isn't affected.
If your DH is as lovely as you say then clearly he's going to try to get work asap - and really, that is all his ex should expect of him. If she also continues to receive at least some child maintenance - thanks to you - then I'm not sure what else she can rightfully demand.
Otherwise, as I said before, she'll need to find her own way through this. I don't doubt this will be a shock to her system - as a large drop in income would be to most people - but unfortunately, in this day and age she must realise that many jobs are insecure ? The other thing to consider is that if she was getting so-called spousal maintenance (i.e. DH being very generous as it wasn't court ordered) then presumably this would have stopped once DSC turned 18 ? She can't possibly have imagined this would have continued forever and especially not when it was based on very different past income ?
If she's mortgage free then potentially she might be able to remortgage for a small sum to tide her over, or she could consider a lodger(s), or she might downsize. Though all of that is of course up to her. What I'm saying though is it's not really much of DH's concern though I appreciate he's worried about possible impact on his child and I understand he wants to minimise that. However - the child is the responsibility of both parents and she needs to do her bit too to avoid any impact. Sometimes though, stuff happens, and however galling it is, you just have to accept that this is how life is for the moment and that things have changed.
Personally, if she's the sort of ex to kick off rather than seek solutions then even when your DH gets a new job, and if he has genuinely disposable income to spare, I'd recommend that he stops this "spousal" element thing anyway. Any spare money could be saved for the direct benefit of the child (uni, car etc) - unless he thinks that ALL the money handed over 0- whatever it's called is actually being used specifically for the child.
I know he doesn't relish the thought but I think he needs to have a very explicit discussion with her about the situation and tell her when she'll get her last "big" payment - obviously it's only fair he does this so she knows where she stands. He should also tell her how much child maintenance you're able to pay on his behalf and impress upon her that you don't have to do this if she starts getting snarky. I think if he pussy foots around her and gives her any sort of impression that things will go back to how they used to be once he gets another job, he'll be storing trouble up for the down the line if a) he ends up with an even lower salary and b) when DSC becomes an adult and "spousal" stops anyway (along with child maintenance for that matter).
I also think you and he need to have a very clear convo about what you're prepared to do and/or not do so no "promises" are made to the ex on your behalf. Bear in mind the impact on your savings and future plans etc which you'll want to try and protect as much as possible. If he gets a lower paid job then that will probably have a knock-on effect on what can be saved for a future move - so it's no good him thinking that the ex's expectations can be magically solved when he gets a job.
Sorry to have written so much, but I think he needs to stand firm here. It's a bit of a watershed moment isn't it ?
Also hope the depression card isn't used to emotionally blackmail him into doing stuff that's inaffordable. I don't say that lightly having suffered from severe depression myself at different times in the past (pre-kids). Obviously, most people would feel worried and stressed by a drop in income but I don't think that'd tip most people into depression just like that unless there were other extreme stresses going on and this was the last straw. I actually think the ex sounds as if she's in a relatively strong position what with being mortgage free - this gives her options many people don't have when they face an income drop. Ditto, she can, in theory (not withstanding the economic climate) look for extra and/or different work because she works P/T - which, for example, someone who was already working F/T wouldn't find it so easy to do if they needed more money. Anyway - I've personally not suffered from depression for many years but when I later had mild bouts this was managed with ADs etc which meant it didn't really impact upon the children I had by then. I know everyone's different, but I think for it to affect DSC their mum would have to be quite severely affected. Hopefully all this won't lead her anywhere like that.