A little tricky. It is hard to guess how bad the bike is -- I've seen some left out that long (in Minnesota) with only a scattering of cosmetic rust, others that were pretty well trashed.
First thing to do is to clean it off. Get a nylon scrub brush or a non-abrasive "scrubbie" and a bucket of soapy water and work it over pretty well. Rinse well with a hose, but DO NOT SPRAY HIGH-PRESSURE WATER AT THE HUBS OR OTHER BEARINGS!
Next, get an old toothbrush and some WD-40 (yes, people will scream that's a no-no, but ignore them) and clean the chain and derailers (if it has derailers). Take care to keep the solvent off the tires when you do this. (I like to lay a section of newspaper against the wheel, tucked behind the derailer.) Then oil the chain and derailers well, wipe with a cloth and oil again. And wipe again. (It's best to use some chain oil from a bike shop, but regular 3-in-1 or some such will do for starters.)
Now check the bearings. Spin the wheels and see if they seem to spin freely and smoothly. Turn the crank with your hand and feel for any roughness as it turns. Sit astride the bike and turn the handlebars, feeling for any roughness in the "headset" that the fork pivots on.
If the bearings seem to check out, give it a test ride. (If not they will need to be disassembled and rebuilt.) Check the brakes, and shifters, if it's a multi-speed. To test the brakes, one should do several stops at various speeds and in various conditions, including a few panic stops.
Here you're very likely to discover that the shift and brake cables have rusted up -- the most likely problem. There are a few ways to deal with this, the most obvious being simply replacing the problematic cables. Short of that you can try to work the cables free, then work some sort of lubricant into them. A trick I like to use is to somehow dribble "oxalic acid" (available from paint stores as a cleaner) into the cables. This actually dissolves rust. (Oxalic acid is also useful for removing cosmetic rust on handlebars, et al.)
After this, you could consider repacking the bearings, but that depends on how much you really like this girlfriend. ;)