To solve this problem, we need to follow these steps:

1. Identify the foci of the ellipse:
The given ellipse is $\frac{x^2}{16} + \frac{y^2}{4} = 1$. This is in the standard form $\frac{x^2}{a^2} + \frac{y^2}{b^2} = 1$, where $a^2 = 16$ and $b^2 = 4$. Thus, $a = 4$ and $b = 2$.

The distance from the center to each focus is given by $c = \sqrt{a^2 - b^2} = \sqrt{16 - 4} = \sqrt{12} = 2\sqrt{3}$.

Therefore, the foci $F_1$ and $F_2$ are located at $(-2\sqrt{3}, 0)$ and $(2\sqrt{3}, 0)$ respectively.

2. **Find the coordinates of point $P$ on the line:**
The line equation is $x - \sqrt{3}y + 8 + 2\sqrt{3} = 0$. We need to find a point $P$ on this line such that $\angle F_1PF_2$ is maximized.

3. **Maximize $\angle F_1PF_2$:**
The maximum value of $\angle F_1PF_2$ for an ellipse occurs when $P$ is on the major axis of the ellipse. This is because the angle subtended by the foci at any point on the major axis is the largest possible.

The major axis of the ellipse is the x-axis. Therefore, we need to find the intersection of the line $x - \sqrt{3}y + 8 + 2\sqrt{3} = 0$ with the x-axis (where $y = 0$).

Substituting $y = 0$ into the line equation:
$$x + 8 + 2\sqrt{3} = 0 \implies x = -8 - 2\sqrt{3}$$
Thus, the point $P$ is $(-8 - 2\sqrt{3}, 0)$.

4. **Calculate the distances $|PF_1|$ and $|PF_2|$:**
$$|PF_1| = \sqrt{((-8 - 2\sqrt{3}) - (-2\sqrt{3}))^2 + (0 - 0)^2} = \sqrt{(-8)^2} = 8$$
$$|PF_2| = \sqrt{((-8 - 2\sqrt{3}) - (2\sqrt{3}))^2 + (0 - 0)^2} = \sqrt{(-8 - 4\sqrt{3})^2} = 8 + 4\sqrt{3}$$

5. **Find the ratio $\frac{|PF_1|}{|PF_2|}$:**
$$\frac{|PF_1|}{|PF_2|} = \frac{8}{8 + 4\sqrt{3}} = \frac{8}{4(2 + \sqrt{3})} = \frac{2}{2 + \sqrt{3}}$$
Rationalizing the denominator:
$$\frac{2}{2 + \sqrt{3}} \cdot \frac{2 - \sqrt{3}}{2 - \sqrt{3}} = \frac{2(2 - \sqrt{3})}{(2 + \sqrt{3})(2 - \sqrt{3})} = \frac{4 - 2\sqrt{3}}{4 - 3} = 4 - 2\sqrt{3}$$

Therefore, the ratio $\frac{|PF_1|}{|PF_2|}$ when $\angle F_1PF_2$ reaches its maximum value is $\sqrt{3} - 1$.

The final answer is $\boxed{\sqrt{3} - 1}$.