nice for you to helping a buddy.
this is the place for lots of informed folks.
So its interesting to see the reaction for the price differential between the 775, mines about a month old while I get my 818 fixed.
my 775 was around $500 with the newegg deal but a X4000 is anywhere from 500 to 700 more.
but the basics here wrt to eliciting comment and advice would include considerations like
room size/geometry and or any attached total volume
listening / viewing preferences
allocations for amp/speakers/subs, screen/TV/projectors/players
most here will help but there is more basic info needed wrt to a more comprehensive and satisfying build with a minimal infection/itch of up-grade-it is.
Once my 818 is repaired, and I see hot hot it runs (I don't expect much) I expect to go for the X4000. with the Emotiva UPA 7 it will be a great combo.
Has your friend viewed the various set-ups folks post about here/
THE BIG ROOM: Mitsy DLP 73837 Xbox1 PS3 slim Panny BD 220
Denon X4000 and Emotiva UPA7 for 9.2
Klipsch F3 FL/R, G-28 C, RB 51's, FW's , RS3II SBL/R, RC3II, SL/R, SLX-FH's
2 30" BF THTLP'S and SubMaximus and Inuke6000DSP