The 250 rebel is a great entry level bike, you may even find the learn to ride classes I hope you enter your wife in use the rebels or something very close to it, its perfect for small framed people up to a 5'5" or so. They are very forgiving and dependable, when she is ready to upgrade sell it for what you have in it kind of bike, and that is what I think is exactly what will happen, she will want to upgrade.
I can't say I agree with getting her a dirt bike, for a guy sure but not for a woman who will most likely go into oncoming traffic before she willingly tackles dirt on or off the road.
Any learn to ride class with driving as well as classroom should be in order, ABATE and MSF courses are available all over, I would recommend you take it as well if you have not yet as it will most likely decrease your insurance premium.
Be prepared that she may not like riding her own bike though, I wanted my wife to have her own since I am a little more aggressive rider and I get tired of her screaming to SLOOOOOOOOOOOOOOOWWWWW DOOOOOOOOOWWWWNNNNN, but it was of no use, now she only whines when I drag the floorboards...
I mean seriously my wife is a better passenger on the R1, I think because she is scared to the point of being breathless....
another link that may help.