If they're coming from San Francisco, they'll most likely be going through Petaluma, not Santa Rosa/Sebastopol. I know there is a Lucky's supermarket (unless it's changed names) in Petaluma - I think the turnoff is for Lakeville Road, which will take you to the Bodega Highway - anyway, it's on the right about a block after you get get off 101 at that exit headed to Bodega. It's in the same center as the McDonald's (which you'll probably see first).
Also, there are 2 grocery stores in Bodega and both are usually well stocked plus they do have some ready-made and deli foods in them. One is Diekmann's (on the bay side past The Tides) and the other is at Pelican Plaza (on the right at the next sharp curve and across from the Candy & Kite store). You can also order meals to go from the local restaurants, and Lucas Wharf and The Tides both have fish markets. Across the bay is the Spud Point Crab Company - not a sit down restaurant (has tables outside) which has great clam chowders and sandwichs.