96 Honda Civic DX manual – axle leak

Working on a friend’s ’96 Civic DX hatch manual.  Couple months ago, he presented with typical right-side CV joint noise, and right (passenger) boot was torn.  Outer, I think.  Axle was clearly beyond re-booting, so I had him pick up a reman axle from Napa.  Replacement went smoothly, in & out of garage in 45 minutes.  Axle popped into diff with a nice normal feel.  Symptoms gone, happy friend.
A few weeks later, he was back with a leaky radiator.  While the hood was up, I checked my work on the axle, and found a ton of MTF all over the area.  Pulled the axle out and measured its sealing face at 35mm.  Verified the OEM spec on the seal was for 35mm.  The seal felt pretty good, not stiff or anything, but maybe a little loose at 35mm.  Sent him to the dealership for an OEM seal and some Honda MTF.  New seal went in without difficulty; I set it flush with the outside face of the diff as my googling indicated, and as the old seal was installed.  I greased the seal with high-temp grease and reassembled.  The seal looked positioned well on the axle’s sealing face.  Drained and refilled the fluid.
That was like 3 weeks ago.  I asked him a while back if he was leaking again, and he said no.  Today, come to find out he’s leaking at least as bad as before.
So… what have I done wrong, what am I missing here?  Thanks!