Apple watch

If it is linked to her Verizon account, it could not be activated with your cell number, unless your cell number is part of your account. Only numbers that are part of her account can be linked or part of her account. If you have your own Verizon account, you need to activate it as part of your Verizon account.

She better be careful. I know of someone that wanted to purchase an Apple Watch, but did not want to purchase a cellular one, through Verizon, but they had a deal on the price. She had not intended to activate the cellular, however Verizon said it was part of the deal and had to activate cellular on her account for the Watch for a set period of time. So, if your girlfriend purchases the Watch, she needs to make sure she does not have to activate it on her account, which would make it impossible for you to activate it on cellular until her term was over. She might be better if the two of you went in together and got it on your account and she reimbursed you, if she is just wanting to pay for it.

That should not be a problem. Until it is activated, it is not associated with any account.

You need an iPhone 6s, SE (1 or 2), 7 or newer running iOS 13 or newer.

If you are on different accounts, she will need to pay you so you can pay your account, since the Watch would be on your account.

She can give you $10 a month, but since the Apple Watch shares the iPhone's phone number, it is going to be charged on your iPhone Verizon account.