JB007 - sorry to hear about your friend's medical problem.
The intention is for NPPL medical requirements to be 'similar' to DVLC ones; I am not really qualified to discuss medical matters, but as I understand it your friend should be able to fly a C152 on the privileges of an NPPL; his medical condition would dictate whether he could also take a passenger.
The IMC Rating question is not as clear. Again, as I understand it, you would need at least a JAR Class 2 medical to put an IMC Rating on an NPPL - if indeed the addition of such a rating is permitted at all.