If he's going to go rogue/wizard, there's no reason to dip swordsage. Dex to damage is nice, but it's not worth losing two levels of spellcasting, and Greater Invisibility takes care of most of his sneak attack needs.
The Divination Mastery thing doesn't 'cripple' your spells, it gives them -1 CL, which is counteracted by practiced spellcaster, so in reality, given that you're going to have practiced spellcaster ANYWAY to offset your rogue levels, you basically just net a free +1 divination.
As for duskblade spellthief etc, i'd really recommend rogue/wizard. Duskblade is kind of okay at low levels but it very very quickly loses puff and it has few of the utility spells that can really make a rogue shine. When you have the BAB to qualify, levels in Abjurant Champion are a good idea as well. Dipping swordsage gets you access to Shadow Blade and Assassin's Stance which help your sneak attack damage, as well as various utility maneuvers including going invisible for a round (!!) and being able to avoid full attack and charges. Typically take it one level before you get to prestige out, as the higher level you take it the better but you don't want to delay your prestiges too long.