If I were her, I'd head straight into a F350 DRW with the diesel. If she's hauling a 16k lb GVW trailer, she's going to really be at the limits of a gas engine. The diesel will make her towing experience much easier and probably safer as well.
You'll need to look up the payload capacities of the SRW vs DRW of the two vehicles. I know that with the diesel F250, 16k is at it's very limit and possibly over it depending on the tongue weight.
The DRW will give her extra security when she encounters crosswinds and when the big rigs blow by her. It'll give her all the extra payload she needs.
If she wants bigger brakes (15.4 inch) and a 6 foot tighter turning radius, then the F450 would be the way to go.
You can check out the ratings using the Ford Towing Guide:
https://www.ford.com/cmslibs/content/dam/brand_ford/en_us/brand/resources/general/pdf/guides/18RV&TT_Ford_SuperDtyPU_Nov27.pdf