I say "Go for it" too. Find a lawyer before promising anything, even if it is a close friend who doesn't mind doing things informally. I don't know of startup attorneys in the Bay Area, but plenty of people here should. (If you don't get any responses, I'd directly email 5-6 YC startups in the area and ask who they use.) Your attorney will probably want to do a few things:

1. Button up your legal structure and organizational documents, if they need work.

2. Prepare a term sheet, subscription agreement, and other paperwork for the investor to sign.

3. Verify that the investor is an accredited investor (which I think means $1M net worth, or $200K salary for the last two years). You can take investment from non-accredited investors, but it is a lot messier, and may not be a good idea.

Money from family and friends is a time-honored way to bootstrap a business. You seem aware of the main risks (good chance of 100% loss, relationships can turn sour).

One more to consider is your independence. When you're working for yourself, you're only accountable to yourself. When you take on an investor, even a minority one, you need to think about your work through his or her eyes. If a better opportunity comes along in 6 weeks, you might not be able to jump on it because you now have a responsibility to your investor.