Also, the +10 bonus is in one-handed, so they are also known to be good at dual-wielding. Just because they have +5 in alteration and destruction does not mean they are all of a sudden magic casters. It allows for a little flexibility in case you were not necessarily looking to be a warrior, but wished to play the Redguard race. Last I checked, One-hand is still considered a warrior/combat skill, so nothing is really out of proportion here. If you chose to use one sword and a spell in one hand, this would work here too.
So... they've decided to change the way the race works since the last game was made. You can argue the guide doesn't mean anything, but when it's Bethesda that puts their official mark on it to explain how THIS game works, I would take it's word over past games. This is how Redguards work in Skyrim, and there is nothing wrong with that. If Bethesda chose to rework them, or add magical stats to the race, that is completely up to them. No reason to be upset about it.
The point that you two seem to be missing is that 99% of Redguards hate magic with a burning passion, especially since the War of Betony. Read up on you lore guys. Redguards do not tolerate magic at all.
Quote from the Redguard in Oblivion:
I'm Trayvond the Redguard, Mages Guild Evoker. Surprised? Yes, you don't see many Redguards in the Mages Guild. We don't much like spellcasters in Hammerfell. Wizards steal souls and tamper with minds. If you use magic, you're weak or wicked. My family didn't approve of my vocation, so I had to come to Cyrodiil for my education. I admit... I still have strong prejudices against necromancy, summoning, and illusion. Profaning the remains or souls of the dead is just wrong. And I'm uneasy about tampering with other's minds and trafficking with Daedra."