dude, she isn't referring to montaron's body...how can she have it if she hasn't done the poisoned man quest yet.
Lady Avalon...I'm sorry but I think the only way to do it is cheat the body back with Shadowkeeper. If you're far off in the game I'd suggest you play it through and start a new game, BG2 is replayble enough so you should still have fun. Then you can do the whole harper thing in the new game
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Melusine, Archbabe of the OHF and the LH
Your voice is ambrosia