There's an interesting thread elsewhere about the effect of interference (by vehicles) on ILS. An interference will result in the ILS unit being unusable and immediately switched off.
Presumably before entering the localiser our pilot (who will not have advised ATC) will have morse identified it. If there is then a problem with the ILS during his approach there will be no way of the ATC advising the pilot of this fact if they (the ATC) are unaware that the pilot is using the ILS.