I will be driving out to San Francisco and leaving a car there for a friend who is moving from europe to SFO for a year.
Unfortunately, he had to change his arrival date and will get there about 2 weeks after I've flown back home.
Is there anyplace in or near SFO where I can park the car for a couple of weeks?

There should be long term parking at SFO, check the airport's website.

There are long term parking lots around SFO, both private and belonging to the airport. The private lots are about $10-12 a day, I believe SFO's long term parking is more, I'm not sure. I know of no place around the airport where you could park for free if that is what you are looking for.

I have heard of some SFO hotels allowing people to park in their lots for free if they stay at the hotel before they fly out. Something to check into.

Several hotels in Mill Brea / San Bruno / SFO will allow up to two weeks parking free with each nights stay. Most of these fly-park packages are not cheap, but most will be a lot less than paying 2 weeks parking fees.