Ask your friend to go down to the shop and test it out for himself/herself. IQ wise for low lighting D3100 is slightly better, otherwise they are similar. E-PL1 is smaller and is a mirrorless m4/3 camera while D3100 is a APS-C DSLR (bigger sensor). Both have loads of lens available (D3100 had more, and I think more third party lens too).
Finally, it all boils down to personal preference. And next time, please read up on reviews for each of the camera first and do your homework, instead of coming here to ask which is better to buy, because different people might like different brand and we can be here arguing till the cows come home and with no, absolutely no conclusion. As I had said more than once, "What works for me, might not work for you." so "choose the camera that you feel best for you."
Style and jokes aside, it depends of the size of the camera he wants as well. EPL1 is obviously smaller.
u4/3 camera like the EPL1 has more DOF (can be good for snap shots and macros) . APS-C sensor camera has less (good for subject isolation and shallow DOF shots).
With the right lens both can have shallow DOF shots, but its easier with a APS-C sensor camera.
There is also the Samsung NX series and Sony NEX series cameras that are EPL1 sized but using APS-C sensors. (not as stylo, though imo)
Er... I don't really understand... what is adapter lens? Was it using an adapter to convert existing lens like the OM mount, 4/3 mount, etc? Well... if that was the case Nikon had adapter that could convert OM, 4/3 and all other mount even EF mount to use. So by taking into consideration of only lens built specially for that kind of mount, D3100 had more lens available.
Nikon's lenses designed to fit further away from sensor than olympus lenses (larger flange distance). So you can fit nikon lenses on olympus with adaptor of right length, but fitting the olympus lenses on nikon body will put it further away than they are meant to be and will lead to distortions, vignetting, etc.
yep.. m4/3 cameras like the oly epl1/2 can take almost all known legacy lenses as the flange distance is half of a std DSLR due to the mirrorless design. C mount, Nikon F/G/AFS, Canon EOS, Leica M, R, even latest I saw was a holga LOMO lens with native m4/3 mount .. lol.. : )
But yep.. best is go down to one of the big chain stores/ photo shops and play with it.. good to read up first before buying. All cameras have trade offs.. : )
If looking at mirrorless camera, there are a few that are excellent,
1) Sony - NEX and A55, A33
2) Samsung - NX system
3) Olympus - PEN series (m4/3)
4) Panasonic - G series and GH series
And true, Nikon's sensor are make by Sony, but a camera is not only about the sensor, there are other things at play too... having said that Sony make wonderful cameras (absolutely loved the A55 - after having have a brief hands-on).
So my advise, like many others here, TS was to ask his friend to go down to the stores and test out all these cameras... or any that was within his friend's budget and see which one he/she like best before committing to buy... instead of asking for advises on which brand is a better buy, because like what I have stated before,