Okey, I've just returned to the mortuary in order to receive Xachariah's Heart. Turns out that the zombie was for some obscure reason hostile towards me. I fear that the DEX upgrade is lost me, but I wonder if there is any method to initiate dialog while the critter is hostile?
It was kind of cheating, but to me the reason for why this if even happened justifies it. I took the dialog file of Xachariha (DXach.dlg) and changed its name to one of the other (useless) zombies which are all over the mortuary. Then I copied the file into the override folder and once I talked to the zombie which the file now was refering to, I ended up in Xachariha's dialog.