= Solution
At nucleosynthesis temperatures, $50\,{\rm GeV}$ particles are nonrelativistic. Their equilibrium abundance is negligible because of <Boltzmann suppression>, so they do not behave as an extra relativistic species contributing its full degree count to $g_*$. A stable thermal <weakly interacting massive particle> generally freezes out much earlier; a surviving relic population with the usual dark-matter abundance remains negligible compared with radiation during nucleosynthesis. \b[In that standard relic scenario the helium abundance is essentially unchanged.]
The mass and weak coupling alone do not specify the relic density. If an unusually abundant stable population contributes appreciably to the expansion density, it raises $H$ at fixed radiation temperature and increases helium by the same freeze-out and survival mechanism as part (g). Decays or energetic annihilation products would require their own specified history. Thus a universal numerical change cannot be inferred from the mass alone.
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