If you really need to be told a plan B you shouldn't start a startup.
Plan B is just start building something and stop worrying about getting other people's approval! If you make something cool, then you will be able to find other founders, money ... etc.
Suggestion: list all options before assigning a "Plan A/Plan B" ranking:
( )Pursue your own business plan and forget about YC
( )Get accepted by Y-C and start your business that way
( )Work for a YC Founder.
Each have pros and cons, and everyone's goals and cabilities are a little different. Assign Plan "A/B/C" in whatever way works for you. For some, working for a startup company will probably be the best experience they can get if the goal is to start their own eventually.
Not all ideas, no matter how world-changing, can be turned into a full company with just one founder and roughly zero cash. For young founders, 15k, partners and a basic support system can make a world of a difference.
edit: The real deciding factor for me though will be his equity equation.
Right, but it's usually possible to find another, simpler idea - even if it's just consulting - that will generate $15k and bring you in contact with potential cofounders or customers.
Plan B is just start building something and stop worrying about getting other people's approval! If you make something cool, then you will be able to find other founders, money ... etc.