When I had similar troubles I practiced 3 against 4 scales (triplets in the LH, 16ths in the RH and vice versa), starting with the hands either one or two octaves apart, depending on which hand was doing the triplets and which the 16ths. At the beginning I had to do it quite slowly to get the rhythm correct and I felt it as a complex pattern between the hands; eventually I was able to speed up and to feel it as clear threes in one hand and fours in the other. After doing that on and off for a year or two I got to the point where I could just sight read through 3 against 4 passages without worry.
As for etudes, the first of Chopin's posthumous ones is all about 3 against 4.
Edit: Oops, not used to quavers, semiquavers, etc. So the student's problem is 3 against 2. So the scales (modified appropriately) work for that, too; and the third of those posthumous Chopin etudes targets 3 against 2.