Low-temperature heat capacity, upper-critical-field measurements, and ac magnetic susceptibility are utilized to study the interaction between superconductivity and magnetic order in the orthorhombic compounds (${\mathrm{Lu}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Er}}_{\mathrm{x}}$)${\mathrm{RuB}}_{2}$. Reentrant superconductivity is observed in a concentration range just below a critical value (${x}_{\mathrm{cr}\mathrm{\ensuremath{\approxeq}}}$0.5), where the superconducting and magnetic phase boundaries meet. For samples with concentrations greater than ${x}_{\mathrm{cr}}$, two magnetic transitions are observed, opening the possibility of complex magnetic states. The strength of the effective exchange field, ${H}_{\mathrm{ex}}$, is derived from the conduction-electron--rare-earth spin-exchange interaction. Superconductivity is quenched and reentrance occurs when ${H}_{\mathrm{ex}}$ becomes comparable to the Pauli paramagnetic limiting field. Heat-capacity data for ${\mathrm{LuRuB}}_{2}$ yield a relatively high Debye temperature of 487 K and an electronic specific-heat coefficient of 7.05 mJ/mol ${\mathrm{K}}^{2}$.