  M dwarfs are common host stars to exoplanets but often lack atmospheric
abundance measurements. Late-M dwarfs are also good analogs to the youngest
substellar companions, which share similar $T_{\rm eff}\sim2300-2800~K$. We
present atmospheric analyses for the M7.5 companion HIP 55507 B and its K6V
primary star with Keck/KPIC high-resolution ($R\sim35,000$) $K$ band
spectroscopy. First, by including KPIC relative radial velocities between the
primary and secondary in the orbit fit, we improve the dynamical mass precision
by 60% and find $M_B=88.0_{-3.2}^{+3.4}$ $M_{\rm Jup}$, putting HIP 55507 B
above the stellar-substellar boundary. We also find that HIP 55507 B orbits its
K6V primary star with $a=38^{+4}_{-3}$ AU and $e=0.40\pm0.04$. From atmospheric
retrievals of HIP 55507 B, we measure $\rm [C/H]=0.24\pm0.13$, $\rm
[O/H]=0.15\pm0.13$, and $\rm C/O=0.67\pm0.04$. Moreover, we strongly detect
$\rm ^{13}CO$ ($7.8\sigma$ significance) and tentatively detect $\rm H_2^{18}O$
($3.7\sigma$ significance) in companion's atmosphere, and measure $\rm
^{12}CO/^{13}CO=98_{-22}^{+28}$ and $\rm H_2^{16}O/H_2^{18}O=240_{-80}^{+145}$
after accounting for systematic errors. From a simplified retrieval analysis of
HIP 55507 A, we measure $\rm ^{12}CO/^{13}CO=79_{-16}^{+21}$ and $\rm
C^{16}O/C^{18}O=288_{-70}^{+125}$ for the primary star. These results
demonstrate that HIP 55507 A and B have consistent $\rm ^{12} C/^{13}C$ and
$\rm ^{16}O/^{18}O$ to the $<1\sigma$ level, as expected for a chemically
homogeneous binary system. Given the similar flux ratios and separations
between HIP 55507 AB and systems with young, substellar companions, our results
open the door to systematically measuring $\rm ^{13}CO$ and $\rm H_2^{18}O$
abundances in the atmospheres of substellar or even planetary-mass companions
with similar spectral types.
