Can I use Ripple to keep track of loans to friends? Say I already lent EUR 100.00 to a friend, in cash, quite informally. Can I use Ripple to keep track of it? If so, how?

I guess I need to get him sign up at ripple.com (or get another Ripple client).

Then I grant him &gt; EUR 100.00 trust.

Then he can just "send me" EUR 100.00.

Is that all? Or is there anything more to it? Does my friend need trust from anybody else in Ripple in order to be able to do that?
Yes, Ripple is created precisely for such purposes. Both you and your friends need some small balance in ripple first to "activate" your accounts. After that, you need to grant your friend trust for at least the amount he will owe you, and then he needs to send you money. After that, you will see a net positive amount on your account, and they will see a net negative amount.

If you don't trust your friend with the money but you are connected through a web of trust, there is still a possibility of them sending you money. The transaction will appear from them, but your balance will show some other friend owes you money, and in part your borrower will owe them money.