The average hydrogen-bond strength in pure water is higher than the average H-bond strength between the first and second hydration shells around a $\ce{Al^3+}$ ion (or any cation, for that matter). This leads to a shift in the OH stretch toward the lower frequencies.
This has something to do with the induced polarization resulting from the positive charge, no doubt, but I fail get why the H-bond becomes weaker. The H-bond would get stronger if the electron density around the H-atom participating in the H-bond gets smaller (larger coulombic attraction between H and O). When $\ce{Al^3+}$ approaches the O-atom of a water molecule, electron density will be forced toward the positive charge, ultimately affecting the H-bonded H-atom, which would give stronger H-bond, at least in my head.
So what's wrong with this picture?