  Using ultrapowers of $C^{\ast}$-algebras we provide a new construction of the
multiplier algebra of a $C^{\ast}$-algebra. This extends the work of Avsec and
Goldbring [Houston J. Math., to appear, arXiv:1610.09276.] to the setting of
noncommutative and nonseparable $C^{\ast}$-algebras. We also extend their work
to give a new proof of the fact that groups that act transitively on locally
finite trees with boundary amenable stabilizers are boundary amenable.
