Besides the noise, you will also get more soot on all of your outdoor surfaces (railings, patio furniture, windows, etc). The wall might block some of that out.
I'm about 4 long urban blocks away and level with an un-walled freeway and we get mostly background noise and very light soot during an east wind (we have prevailing westerlies, thankfully). Occasionally, we hear unmuffled cars and loud motorcycles zooming around or sirens late at night, when things are otherwise quiet.
During the day (when most buyers are looking at houses) the normal neighborhood noise of buzzing lawn mowers and warming up boat engines and humming pool pumps and such cover any freeway noise. But, we are open-doors-and-windows kind of people and haven't run our a/c yet this year (it's only been in the low 80's for Pete's sake), while our next door neighbors almost never open anything up (i.e. their a/c compressor is our major white noise). They probably don't ever hear a thing (but that might be weed-induced delirium - O.T.).
So what am I trying to say? If your friends are outdoors-entertaining, open doors & windows people, they will probably be unhappy for however many years they decide to live in this house. If this house is a great bargain, there might be an obvious reason for that, and will remain when they try to sell. The only reason I'd think about buying it myself, was if it had some very unique factor - view, water, exclusive neighborhood or the best schools in the county that everyone wants into, along those lines. Note: price is not "unique" if it's going to make you miserable or adjust your lifestyle considerably.
I wish them luck.
Q
Blue
Q