NP Complete Proof - Polynomial Reduction
Probably you can get your answer from here: Reducing independent set to triangle-free subgraph
Basically in order to prove that $TFS$ is NP-complete you can just check the first proof in the link's answer. Given a graph $G$ with an independent set $k$, there exists a $G'$ with a $TFS$ of size $|V|+k+1$.