Ok 11 feet apart is not necessarily far apart if he sits 20 feet away. Knowing more about room dimensions and where his seating position is would help a lot.
Unlike Mike, I normally do not like speakers that are farther apart than I sit away unless I have I have no choice or I listen mainly to classical where scale and a big stage is more important than centre focus.
So, back to 11 feet apart. I would generally prefer to sit 11-13 feet away (centre of your head to the tweeter) as long as the 11 feet is the distance between centres. 13 feet for vocal and rock, 12-11 feet for a good variety and with classical only - Mikes formula makes sense (to me that is).
If the room is narrow and the speakers are very close to the side walls I suggest having the speakers cross in front of the listener rather than the norm being behind. Easier if the speakers were a box (S3’s being curved), however much you see of the inside of the speaker, toe the crap out of it so you see the same amount but this time, on the outside. It will look odd but... this will deepen the soundstage (but narrow it), lessen side wall interaction (early refections) and make for a much wider sweet spot if listening is not a loner sport for him.
However, after all this rambling on, I do not believe in any room acoustic rules, it is more of an art than a science. Every move you make offers gains here and losses there. It being all about compromise means its about choosing the compromise that best suits his or hers wants and/or needs.