anyways, so, its 360, uhh, is there a way of putting a mod on there? or scripting?
Nope. Sorry. There is no way to easily (or legally) get user made content onto the 360 version. Even getting what you need to do so will void the Xbox's warranty and get your fiance to be banned from XBL.
Mods will, however, work fine with the steam version (I use the steam version and it is functionally identical to the retail version).
You download the game, then you plop whatever mod you want into the FO3/Data folder, tick it off with the data files manager from the launcher, load up, and play. Or you can use FOMM (Fallout Mod Manager.) But since you're just going to use this one very special mod initially, you don't most of what FOMM offers.
Also, your idea to have Three Dog say your fiance to be's name won't work unless you have the guy who played 3Dog, or someone who can do a suitable impersonation to record the new lines. You're probably better off having it via note or something.
Or, for extra cute points, if you have a decent microphone, make a character of yourself who looks as much like you as possible (get the beauty packs/hairstyle packs if necessary or desired), then voice your own proposal and have the digital you propose.
Hmm.
Edit:
does the PC game have a record option in "console?" or something like that?
Record? As in output a gameplay video into something you can plop onto youtube? Not that I'm aware of.
Anyway, to make mods, you'll need to download the GECK first. http://geck.gamesas.com/index.php/Main_Page has what you'll need.
Don't hesitate to ask for help with scripting and such in the GECK http://www.gamesas.com/bgsforums/index.php?showforum=46.
Edit2:
NOOOOOOOOOOOOOOOOOOOOO. lol. thennnnnn is fallout 3 expensive for the computer? i will encourage him to play it, and then tell him there are some crazy things that can happen, like diff mods. PFFt. lol
Nah, it's not expensive. Retail, it sells for around ~$10 less than the Xbox version ($50 versus $60), but that's before markdowns due to age.
Steam has it full price still, but that offers you some secrecy (no disks lying around), and offers the convenience of steam.
Anyway, just tell him that the PC version's mods make the game vastly better in every possible way. Because that's the truth.
Although, you might want to make sure his rig can run the game properly. What are the specs on his PC?