Why does the helium hydride ion have such a large dipole moment?
The dipole moment of an array of charges is defined as
$$
\vec{\mu}=\sum_i \vec{r}_iq_i
$$
where $\vec{r}_i$ is the position of charge $q_i$ in the molecule with respect to the center of mass. In a neutral molecule the sum of the charges is zero and therefore the terms in the sum partially cancel. For ions this is of course not the case and the dipole moment is much larger than for the corresponding neutral molecule (provided that the ion is not completely symmetric such as H$_2^+$).