You will get an idea if you look at the gauge or tension. Can't remember offhand what 4ply, DK etc is for crochet but any modern pattern will say. If it's for a babe then 4 ply is most likely. In any case, your wife would be well advised to try a tension square in her chosen substitute yarn and make any adjustments to the pattern to have even half a chance of ending up with something resembling the picture :) Sorry if this sounds bossy - I have made too many mistakes in the past!