Helton's teammate, Larry Walker, has the biggest BA home/road difference I can find. Walker was (.348 - .278) = 70 points better at home than on the road, which is a bit odd because Walker doesn't even have half the Coors Field games that Helton has.
Another Rockie, Dante Bichette, has a 59 point difference in his h/r BA, which would also surpass Helton's 58 point difference. Red Sox 2B Bobby Doerr had a 54 point difference.
There might be one or two more out there but I checked most of the guys with notorioiusly massive tOPS+ career splits.