Under JAA, the predecessor of EASA, the 45 (not 50) hrs for a PPL could be reduced to 35 if you had a microlight licence with at least 100hrs PiC in 3-axis microlights. I don't think that rule survived into the creation of EASA, but is probably what you're thinking of Jan. I was able to exploit that when I got my JAR PPL(A) in 2001, having a few hundred microlight hours at that point.
I think that our holder of an fATPL is in a bit of a different place 'though, as he already has an ICAO compliant professional licence - the question is about the hours in micro/ultralights counting towards the 1500hr requirement for removal of the frozen aspect of his ATPL.
G