You kind of need to know the person. You can also share mini-games that you buy from PSN (like that arcade hockey or arcade football).
Anyways, what you do is:
1) Person A buys the "Downloadable content"
2) Guy B makes a new account on his own ps3
3) Person A signs into PSN on Guy B's new user (all of his DLC is now on this friend's ps3)
4) now his Guy B's ps3 has the DLC, and it works for every user account (not just the one Person A signed in to)
Unless you are going to pay your online friend somehow, and he's going to tell you his PSN e-mail/password, you probably need to know the person in real life to do this. You can split DLC five-ways (the person who buys it and then 4 of his friends).
EDIT: But I'm not completely sure if this will work for an update sort of thing. I've done it for those $10 or $15 mini-games, but this may be different. Worth a try, nonetheless.