Okay please imagine with me Ashe's reaction when Dedue shows up alive. Based on what Ashe says to you it's clear he's pretty torn up over Dedue's supposed death. And after losing his parents, his big brother, and then a parent again, he's probably touchy over the issue of death. So here he is grieving, thinking of his friend whenever he goes into the greenhouse. And then Dedue shows up, a little worse for wear, but very much alive. Ashe would be hysterical and I cannot be convinced otherwise.
Oh he's so happy, like you wouldn't believe! All decorum goes out the window, he full on tackles Dedue and even manages to move him back like two inches