Solvents generally don't work all that well on latex based paints. Seeing as how your SO used a spray paint, I assume that the paint is probably an enamel of some type, on which solvents will work. The degree of efficiency will be determined by how long the paint has been left to dry. And yes, most (if not all) solvents that will remove the paint, will also remove any vegetation.
IMHO, your best bet is going to be a good pressure washer. I've had very good success with removing all different types of paint from concrete sidewalks with one.
ETA: Missed your post about it being latex. If it was very recent, a wire brush may do just fine. If the paint has already cured, a pressure washer will definitely do the job without killing the grass.
Last edited by JBDivmstr; 08-31-2011 at 06:34 PM.
Reason: Missed post.