Ok, lemme answer all of the questions in 1 shot.
1. ESF needs HL1 because it is built on the half life engine. As in it needs the HL source code to run. It's basically what makes all of the extra code, models, etc. of ESF work.
2. If your friend wants to give you his HL, sure, whatever, it'll work.
3. Half life is really, really cheap. Like seriously, $10.
It's online only. The only offline play is against bots.
-Karrde-