Very confused 4-string Steinberger Spirit owner here: mine has a zero fret, and no nut.
If you're looking at the same type as I have, there shouldn't be any real problem with stringing it left-handed, though you may need to stick to a light gauge.
Another option, if you're not wedded to double-ball strings, is an adapter for single-ended strings. They're cheap, and there's no modification involved because it's just held on by string tension. Mine has the same sized hole for each string, so that's another option.
See how the grooves widen from G to E string? This would stop him from simply swapping the strings around. You can't reverse this like a normal nut because it's not "flippable" like a normal nut is. Even with the single ball end string adapter, the strings still have to fit down this channel. Of course the OP can learn to play his strings upside down, That's what I do.