A lot of the traffic paint used today is acrylic, though some areas still use alkyd/solvent based product. In some areas they still use yellow with lead in it, because it is a little less expensive than the lead free yellow. If it is acrylic have him try a product called Dirtex, but make sure it is the aerosol version not the powder. It is a general purpose cleaner, but it is the painters' secret for overspray removal. It should be readily available at a hardware or paint store. If that fails rubbing alcohol will soften acrylic paints also. If it is solvent based he will probably have to gamble on various solvents, starting mild and working up in strength, with the risk of hazing or other damage to the factory paint, ie lacquer thinner, xylene, MEK. There are also some very effective citrus based solvents, such as Dissolv-It Contractor's Solvent which may work on either water or alkyd coatings and generally will not harm the factory finish.