I will tell you the easy fix. If this does not work then you will have to remove what is loose and replaster. But first try this: paint the soft plaster with some acrylic like Acryl-60 by Thoro Products. Let it dry it should take less than half a day this should reconsolitate the surface but plaster does not bond well to the acrylic so you need more treatment. If it does not reconsolidate the surface then take the plaster off but I am guessing it will. After the Acryl-60 is dry paint the area with some polyvinyl acetate (PVA) bonding agent. Let this dry then spread a coat or two of some setting joint compound like EasySand by USG. If the surface offset is deep you could use some Durabond for a fist coat. This does not sand easily so when you get close to flush use the EasySand. If it takes more than one coat you can put a second or third coat on as soon as the prior coat has set. When you are flush then let everything dry a day and sand it, touch up any imperfections and prime and paint. After you get the Acryl-60 on you can tell if it consolidated the surface. If it did not then go no further and instead remove the damaged plaster down to the lath and post here for further instructions. Alternatively, you can visit www.oldhouseforums.net/ohfbbs where some old time plasterers hang out and where the procedures for patching have already been posted.