Well, there is one form of birth control that's both free and 100% effective.
And although the various safety measures rock climbers use sometimes fail, there is a method of rock-climbing safety that's 100% effective: not going rock-climbing in the first place.
And I have a perfectly safe car: that is, I never leave my house.
I take it you can see how ridiculous these are. The point (well, one point) of cooking is to be able to eat food and not get food poisoning. The point of all the ropes and things is to be able to climb rocks and not fall to your death. The point of seatbelts and crumple zones and antilock braking is to be able to drive from place to place and not get killed in a car crash.
Not having sex -- which I assume is what you are referring to here -- isn't a form of birth control, because it doesn't accomplish the objective of having sex and not making babies.
It might still be a good idea, of course. Just as it might be a good idea never to go rock climbing (maybe it's just too dangerous to be really worth it for anyone who's thinking clearly) or never to drive a car (it's bad for the environment in lots of ways, after all). But that's a separate argument, and the case needs to be made honestly (cost/benefit on failure rates, appeals to alleged rules handed down by alleged gods, etc.) rather than by snarky one-liners like the one above.
You don't need to have sex. Abstinence is a form of birth control as much as being bald is a hairstyle.
I know that this may sound strange, and I don't share this trait myself, but for many people sex is a critical part of forming a romantic bond with their partner. Without sex, they feel unloved and invalidated in the relationship. That's just part of their emotional needs. So for some people sex is indeed a psychological need.
But a large fraction of people would find their lives made much, much worse if they were unable to eat solid food, to leave their house, or to drive a car.
Exactly the same goes for sex.
Brief reminder of the context for this discussion: the person the OP is about had a child despite not having much money, various people here said you shouldn't be having children if you're too short of money, others pointed out that not having children is sometimes difficult, and that is when user rubidium suggested that total abstinence from sex might be the answer.
So we're talking here about whether it's reasonable to say that poor people should just never have sex.
I guess opinions on that might vary. My opinion is: duh, no, what an absolutely terrible idea. You don't literally have to have sex any more than you literally have to have friends or holidays or music, but just like any of those it's a hell of a thing to say whole classes of people should just do without.