What about

CompleteQuest 000e790f

Sometimes things are easier than you think
and in case you're wondering... Yes i did register just to post this

You just told us that your post was after you removed the bomb. The mod that he had you create alters the dialogue of the guy who you talk to after you disarm the bomb. What you need to do is to fast travel away from the monorail in your save. Trying to walk back will result in it exploding. Then you can go back and report your success and it will allow you to finish the quest without the monorail exploding.

I've got it, if anyone is interested. You will need the GECK game editor for New Vegas.

1) Open GECK, go File>Data and tick the checkbox next to "FalloutNV.esm". Do NOT click 'Set as Active File'. Click OK and wait for it to load.
2) In the Object Window, expand 'Actor Data', then click on Quests. When they load, find the quest with the editor ID "VMS08". Double click it.
3) Click on the tab button labelled "Topics". It should open with "GREETINGS" selected. If it doesn't select "GREETINGS" on the left.
4) The 7th item ought to be "What happened?" (this is the greeting Hsu uses after you've reported the spy, but before you're reported the monorail blowing up). Click on it once.
5) At the bottom of the window is a text area labelled "Result Script (Begin)". Paste the following into it:

6) Click OK on the bottom-right of the window.
7) Go File>Save and make up a name for the file, it's not important. Make sure to save it in the folder GECK suggests (the Data folder under your Fallout NV game directory), and make sure to keep the ".esp" extension.
8) Close GECK. Open the Fallout NV launcher. Click on "Data Files". Tick the checkbox next to your new file, whatever it may be.
9) Load your game, and speak to Hsu. Just activate him, hear the greeting, and say Goodbye. Do not say anything else to him. Do not speak to him again until you disarm the bomb.
10) You should now be able to disable the bomb. It's in the air vent at the front of the train. Disable it and report to Hsu.
11) Exit the game. Open your Fallout NV game directory, go to the Data folder, and delete the mod file you created. Be careful not to delete anything else. Your file should be about 3 kilobytes.
12) When you next load your game, you should get a warning about missing content. That's fine, click OK. If you don't get the warning, you may have done something wrong.

All done!

Since you killed the spy, you should be in possession of a convenient NCR uniform to get past the monorail guards with. Your NCR reputation should also raise from 'Neutral' to 'Appreciated' (or whatever the appropriate jump is if you've aligned yourself differently).