(I'm sorry but any response to "how was your day" other than "fine" or "good" or whatever is a red flag.) He's just not being cool.
There's no need to "engage" Claire at all. Just sit down, shut up, and listen.
This is pretty interesting to me because you are the only one (sorry if I'm wrong and I missed someone's comment) that has framed this discussion around the fact that Claire's statement was made in response to a routine "how was your day" question. My conclusion isn't quite the same as yours, though.
It is true that it is unusual in general for people to respond to that question with anything other than some variation on "fine." It's practically expected that you do so.
But I think that this fact leads to two possible interpretations of any other response, depending on context.
1. It's a dramatic departure from the norm that requires immediate close attention.
2. It's just a more elaborate version of the throwaway response "fine" that requires no more attention than normal.
The reason Marten may have taken the second interpretation would be the fact that Claire has said before that she was stressed, but brushed off Marten's concern on those occasions. So it just seems like more of the same - a throwaway comment as before.
Confession time: I have been guilty more than once of not paying attention when my partner was trying to tell me something, and it has upset her more than once. So I'm very conscious of the problem, but I'm also aware that in a relationship, you aren't normally 100% focussed on your partner at every moment of every day, and you're not necessarily expecting your partner to do that either. Conversations between partners in a relationship vary from throwaway all the way through to matters of the heart, and while you do learn quickly which is which (I'm getting better at knowing what responses to my how-was-your-day greeting deserve a drop-everything-and-listen response), mistakes happen when your head is elsewhere.
I don't think anyone is the devil in this situation. Marten should have listened but I can understand why he didn't this time. He's fallable like all of us. Hopefully this will quickly be sorted out.
Edit: Todays comic - it looks like it will, doesn't it?