To answer the original question: An appropriate river PFD does both provide additional safety, and is also more comfortable. The 'ski' type III lifejackets that you are referring to that are available at Wal-Mart, Costco, etc... are commonly rated with 11lbs of flotation. The Stohlquist, NRS, Astral, etc.. PFD's that you refer to will typically start with 16.5-17lb of flotation. There are also higher flotation rated PFD's available with flotation upwards of 20lbs, these should be considered if you are likely to be dumped into large hydraulics, or rivers of a continuous nature where more flotation is required. The whitewater PFD's not only provide more flotation but also commonly have a greater range of movement allowed. Especially PFD's geared toward kayakers like the NRS Ninja (my personal choice, though I float a raft).
Which brings me to the other question by the OP. I think that your g.f. will be just fine on the Grand Rhonde. There are not many, nor large rapids that she is likely to be swimming in that vest, and so it probably is not necessary to purchase a more suitable whitewater PFD. However, if you plan on going on more adventurous rivers, I would strongly recommend upgrading your PFD's. There is nothing that will put your g.f. off floating a river with you more than being in the water in a Class III rapid and having her PFD keep her head about 3 inches below the water surface for 10-15secs before she washes out. Kills all the joy out of the experience!