The short answer is that they’re caught by the electromagnetic field surrounding the nucleus.
Let’s simplify to hydrogen atoms, protons and electrons. After all, in the period Hawking’s talking about most of the nuclei were just hydrogen ones, i.e. protons. So you have a proton and an electron passing each other. What can happen ?
Firstly, nothing. In fact, under most circumstances that’s what most likely to happen.
Secondly, as you know, the proton can absorb the electron and become a neutron. We’ll come back to this below.
Thirdly, the proton can capture the electron and become a hydrogen atom.
Let’s think about this third process in reverse. What’s a hydrogen atom ? It’s a proton with an electron “going round it” (we won’t worry about the awkward details here), held in place by electric forces because both carry electric charges. This is stable. In particular, the electron doesn’t have enough energy to escape from the “clutches” of the proton. Shine a light on this atom. Light is made up of photons. If one of these photons is absorbed by the electron, the latter can get enough energy to break free from the grip of the electric force holding it around the proton. You can wind up with a proton and a free electron rather than an atom.
atom + photon -> proton + electron
This is ionisation. But the opposite, called electron capture, can also happen
proton + electron -> atom + photon
The electric force is stronger close to the proton, so you can roughly think of this as an electron finding itself close to one, falling into the grip of the electromagnetic field around it, getting caught and having to emit a photon.
As I say, I’m simplifying. A correct description is much more complicated, largely because a better picture of the electromagnetic interaction between proton and electron is them both emitting and absorbing lots of photons.
Yes, both these processes (ionisation and electron capture) are commonly observed in labs.
As Hawking no doubt mentions, there are 3 known sub-atomic forces: electromagnetism, strong and weak. (There’s also gravity, but it’s too weak to be relevant in such processes.) Electrons don’t feel the strong force, so the only way they can interact with the proton is via the electromagnetic or weak forces. In the example of electron capture, the electron is interacting with the proton electromagnetically. As you realise, it can also interact with the proton by being absorbed by it, changing it into a neutron and a neutrino. This was my second possible outcome, the one I promised to come back to. The key point is that this is an example of a weak interaction process.
In line with the name, weak interactions are, well, weaker than electromagnetic ones. As a general rule and as you might expect, the weaker the interaction, the less frequently it happens. So before the weak interaction gets a chance to grab the electron, the electromagnetic one has already caught it.
The details of the weak force are also such that the electron would have to get much closer to the proton to feel it. Once caught electromagnetically and “going round” the nucleus, the details of quantum mechanics tend to keep the electron away from getting that close. So once captured, the weak force doesn’t get a chance to interact with the electron.
There’s a twist here. In certain atoms the electrons are allowed quite close to the nucleus by quantum mechanics and electromagnetism. The result is that occasionally, one does interact with a proton inside in just the way you suggested. This gives rise to a special, fairly rare form of radioactive decay, also called electron capture.