"It's a place where she spends most of her free time"

"It's a place she spends most of her free time in"

Is this sentence grammatically correct?

As stated in one of the other corrections, we do not end sentences in prepositions in English (at least, we shouldn't!).

You could remove the "in": "It's a place she spends most of her free time.",

phrase it with a "where": "It's a place where she spends most of her free time.",

or give the "in" an object: "It's a place in which she spends most of her free time."

"It's a place she spends most of her free time in"

Is this sentence grammatically correct?

I would take out the "in." We aren't supposed to end sentences with prepositions in English. I hope that helps. :)

"It's a place she spends most of her free time in"

Is this sentence grammatically correct?

It's the place where she spends most of her free time