The higher the number the thicker the string. so 10 gauge is the thinnest string and 46 is the thickest. 9 -42 means that the strings are slightly thinner than the previous set you mentioned (I.e the thinnest string is 9 Gauge and the thickest 42 guage) . Much depends on the style of music and personal preference. Thinner strings may be good if he plays rock or blues and string bends. (Literally pushing the strings up while playing to change the note).
If he has an old string packet around you may be able to see what he uses or just ask him to explain all about it in an interested way then ask what he uses.
Sometimes you will see strings described as extra light, light, medium or heavy light being sets at the thin end and heavy being the thicker end of the scale..