From what I've read, the half bet is only used in Limit (or spread limit) games, but since you ask for amounts, I assume you mean NL, where they normally use a full bet rule.

Here's how I read it:

1) The bet is less than a blind, and is all-in. This should be viewed the same as a limp, but creates a side pot, only allowing the player $15 per person in the pot. The next player can do as they please, as it is not a raise, merely a call.

2) Player 2 has raised less than the minimum, and does not constitute a complete raise, it is merely an oversized call. The rest of the table can call or raise this bet, except the first player. If everybody calls or folds, then the first player can only complete the bet, the action is not open to him again.

3) The first player can raise in this scenario, as the action is open to him again under the half bet rule.

Betting in poker - Wikipedia, the free encyclopedia

Originally Posted by Gorgeous

Nice to know that, I've actually never encountered this.

It's a rule to know, especially for live games in a serious setting. You can help a dealer settle a dispute. But In a friendly home game, it may actually cause a brawl, since it's confusing as hell.