This is from the Section 9.6, page 351 of "Classical Dynamics of Particles and Systems" by Thornton and Marion.
By setting a up a system where mass 1 has initial momentum $m_1 u_1$ and mass 2 is at rest.
If we let
$\psi$ be the scatter of angle of particle 1 from the line connecting $m_1$ and $m_2$ prior to contact in the Lab reference frame
$\xi$ is the scatter angle of particle 2 from the same reference line as above in the Lab reference frame
$V$ be the speed of the center of mass in the Lab reference frame
$v_1$, $v_2$ be the final speed of particles 1 and 2 respectively in the Lab reference frame
$u_1'$, $u_2'$ be the initial speeds in the center of mass reference frame
$v_1'$, $v_2'$ be the final speeds in the center of mass reference frame
$\theta$ be the scatter angle of particle 1 in the center of mass reference frame
Then from the fact that:
Conservation of momentum and kinetic energy implies $u_1'=v_1'$ and $u_2'=v_2'$
$u_1 = u_1' + u_2'$ is the relative speed of the two particles before collision, which are the same in both frames
$u_2' = V$ since vectorally, the vectors have the same magnitude but opposite directions
$u_2 = 0$ since the 2nd particle is at rest in the Lab reference frame
Through algebraic manipulations we can write $v_1'$ and $v_2'$ in terms of $u_1$
and through geometric inspection, we can determine that: $$tan\psi = \frac{v_1' sin\theta}{v1' cos\theta + V}$$ which (since $v/v_1' = m_1/m_2$) simplifies to: $$tan\psi = \frac{sin\theta}{cos\theta + (m_1/m_2)}$$
and that: $$tan\xi = \frac{v_2' sin\theta}{V - v_2' cos\theta}$$ which (since $V = v_2'$) simplifies to: $$tan\xi = cot\frac{\theta}{2}$$ implying: $$2\xi = \pi - \theta$$
Then in the case where $m_1 = m_2$ we have: $$\psi = \frac{\theta}{2}$$ so that: $$\xi + \psi = \frac{\pi}{2}$$
Despite all of this fancy mathematical acrobatics, the result seems counterintuitive to what we expect. Why does the scatter angle between the two particles need to always be 90 degrees?
What if for instance, particle 1 strikes particle 2 directly so that particle 1 bounces straight back, while particle 2 is pushed straight forwards, creating a scatter angle of 180? Isn't that what we normally see in billiards (where the balls are approximately equal in mass).