As stated, what you want to do will work. I'll just explain the "WHY" it works.
Secondly, and this is less of a problem because she already has an active Verizon SIM, Verizon will not activate any iPhone that was not originally intended for their network - unlocked or not. So, you have to use an already activated Verizon SIM to get service for non-Verizon phones.
in the last week of august 2015 verizon came out and said we do not care anymore about non-verizon phones. we want to make it easier to bring unlocked phones to verizon. they are activating every single iPhone 6, 6 plus, and google nexus 6. you can check your IMEI/MEID on their BYOD page and it will say it is compatible.
i wrote several of those trade in companies asking them why is there a price difference between unlocked phones, AT&T phones and unlocked t-mobile phones , now that verizon has officially said all the iPhone 6 and 6 plus series phones are equal now. but they are all greedy people who want to find some way of knocking your quote down as much as $125 because you bought they feel there is some difference between individual models
however the bad news is this is not the case with the 6s or 6s+. they don't seem to be recognizing the a1688 or a1687 unlocked models as being compatible with their network. they say we will try our best to activate it if you bring us one, but over the website they claim it won't work. this is the same deal with sprint, my iPhone 6s+ full price t-mobile phone came back as not a sprint device
i decided to keep it. i wanted to switch it out. because i felt the AT&T version might be white listed. but thats not what apple says on their lte page. so i honestly don't know what the heck is going on here
my best guess / theory is sprint and verizon do not activate phones that do not belong to them during the first 90 days
or like you said originally intended for their network
