As I said; Tervigons and Termagants don't do a whole when your meta gets its act together and starts playing with a Lascannon or two in every slot and Melta/Plasma on every infantry unit. Your Tervigons start dropping like flies (and thus, so do your Gants), and then your Ld6 Gants start getting Tank Shocked off the board.
None of the Tyranid Troops work well on their own. It's a giant flaw in the Codex, but it's true. Tyranids require more synergy than other armies because that's the way the 'dex is written (i.e; poorly).
Tyranids and Tau are well-known to dominate the casual (usually all-Infantry/Hybrid lists and never Imperial Guard) meta-game, so I don't know what your friend is doing wrong.