That's a hard one Crystal, but if you're going through a tough time, you're not alone. I just lost one of my Chorus frogs, Chrome
I got my Gnag and Bumpy at the same time (Sept 24th) as you got Banjo, so all it takes for me to know how you feel is to imagine having to make a decision like that with one of those two. Maybe see if anyone here on the FF lives in the Massachusetts and is willing to take her in? That, or find an animal shelter. I honestly don't think you should let her go. It'd be such a shock from living with you for so long and then suddenly being placed back outside. It's entirely your and Banjo's choice, though, and no one else.
I'd at least take her to the vet before letting her go (if you do let her go), and see what's up with her.
Are there any nearby Provincial parks or large forests? I think those would be her best chance if you decide to let her go.
I'd take her in if I lived closer
As for a pro and con list, here's mine:
PROS OF:
Keeping her:
She may likely heal from her current state, even if it requires help from a Vet.
She'll be happy staying with you, and you'll be happy with her staying with you.
She'll be under the care of a very caring pet owner.
No threats of predators attacking her or vehicles hitting her.
She may live MUCH longer than she would in the wild.
Letting her go:
She'll live the life nature intended her to have.
She may heal from her current state.
She may get the chance to breed and have young.
CONS OF:
Keeping her:
A grumpy and/or disagreeing Bf.
She may not heal from her current state.
She will not reproduce (unless you get a Bf for her too

)
It will cost money, though not more than the current cost.
Letting her go:
She may be run over or eaten by predators.
She may die due to shock from the sudden move or unknown location.
She may not live as long.
She may be abused or attacked by humans.
We'll miss hearing about her on the Frog Forum.
I really do think you should keep her, you two are so close. But, like I said, it's entirely up to you two.