"because she already knows what causes of her depression are and does not see any use in debating them further as they only bring her down."
Hard to advice without further knowing details, but I could relate in a way, that the global state of the world causes me depression, but I cannot change the root cause. But I can change my attitude towards things. Deal with the problems I can solve and accept the rest. Spot patterns, where I indulge myself into misery and choose a different track (if possible).
And if we are talking about a childhood trauma, this person wants to avoid, then you can compare it to a splinter that is still in the flesh, causing misery and infection - removing the splinter will hurt and it will bleed - but after the splinter is gone, the wound can heal for good.