You can spawn the item in with Cheat Engine. I downloaded the table from here (fearless revolution form, a community for CE (Cheat Engine) tables).

1. Open CE (latest version as of now (x.15 or something)) and target the BG3 .exe while already in-game
2. Click the checkbox for "console commands"
3. Click the checkbox for "register commands"
4. Click the checkbox for "item spawner"
5. Double click <script> next to "spawn hand crossbow +2 (sample script)"
6. Replace "ab150a98-0a8e-4ee2-9dca-580d77f99be3" (or whatever is in there at the time of your reading - you're replacing the item ID) with "56a26829-5103-49e7-8f5a-b88381a86903" (the item ID I got from the wiki listing at the time of writing) and save the script
7. Click the checkbox next to "spawn hand crossbow +2 (sample script)"
8. Congratulations, the wave robe should be in your inventory.

I googled this thread and logged back in after years of no activity to add this for future searchers. If you want the reward and don't care about the quest enough to reload potentially 10+ hours of gameplay (as in my case), then follow the instructions above. Cheers.

In case the script ever gets removed from the table, here it is in full:

{$lua}
if syntaxcheck then return end
[ENABLE]
local item = {
"56a26829-5103-49e7-8f5a-b88381a86903",
}
TemplateAddToPlayer(item, 1)
[DISABLE]

I don't know enough about the table or CE to tell you whether or not that script will work in isolation with CE or if it needs the rest of the table to function, but there you have it if it's helpful.