Using radial-velocity data from the Habitable-zone Planet Finder, we have measured the mass of the Neptune-sized planet K2-25b, as well as the obliquity of its M4.5-dwarf host star in the 600-800MYr Hyades cluster. This is one of the youngest planetary systems for which both of these quantities have been measured, and one of the very few M dwarfs with a measured obliquity. Based on a joint analysis of the radial velocity data, time-series photometry from the K2 mission, and new transit light curves obtained with diffuser-assisted photometry, the planets radius and mass are $3.44pm 0.12 mathrm{R_oplus}$ and $24.5_{-5.2}^{+5.7} mathrm{M_oplus}$. These properties are compatible with a rocky core enshrouded by a thin hydrogen-helium atmosphere (5% by mass). We measure an orbital eccentricity of $e=0.43 pm 0.05$. The sky-projected stellar obliquity is $lambda=3 pm 16^{circ}$, compatible with spin-orbit alignment, in contrast to other hot Neptunes that have been studied around older stars.
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