Miriam-Webster indicates that the word "freer" is a proper form of the word "free." Although the word "freer" may sound slightly awkward, it is, in fact, grammatically correct.
"Freer" can be used as a form of the word "free" in two contexts. "Free" is an adjective describing a person, place, thing or idea that has the quality of being free. "Freer" can be a comparative form of "free," meaning that someone or something has more freedom than another. "Freer" can also be used as a noun, meaning that the "freer" is the person or object that frees another.
The only context in which it would be unacceptable to use "freer" would be in the case of monetary value. Since the word "free" in reference to cost means that something costs zero dollars, saying something is "freer" in this context would be incorrect since it is impossible for something to cost a negative amount of money.