With PS3, just give them your username and password. Tell them, after they log on to your account, to go to PlayStation Store. On the top right, there's a "View Downloads" icon thingy. Tell your friend to click on that, and re-download First Strike - on his PS3, of course.

I don't own an xBox, but I heard it's the same general concept.

The post above method does not work unless you trade Xboxes, and fix gamertags accordingly. So meh. D: I've tried.

The only way to make any downloadable content work on Xbox downloaded trough a friend's gamertag is to keep his/hers gamertag or play on his/hers Xbox, while he/she plays on your Xbox, with his/hers gamertag.

If you get a new Xbox and recover gamertag and redownload all downloadable content you had, you'll only be able to play with the downloadable content on the gamertags you had during the original purchase.