You
might be able to use a "netter" atomizer on the RN4081, but I've never heard of it being done.

However, there is
another atomizer (and a cartomizer) that will work:
The DSE103 atomizer (discussed
here) is fully compatible with the RN4081 ... and there are now 4081
cartomizers.
I wish I could suggest a 4081 battery to 510 atomizer adapter, but I don't know of any. The only 4081-related adapters I know of go the other way around (i.e., for using a 510 or 901 battery with the 4081 atty).
And then there is the Alexandrian Solution to that Gordian Knot: Suggest that your friend
forget the couple RN4081
batteries he bought a year ago ... and
buy an inexpensive 510 kit to try out. [
This webpage lists some inexpensive sources. Or, for only $10, he could try the
Q-cig 510 clone.] If he likes the 510 atty, he can always upgrade to your eGO batts later on.