The meaning is not straightforward but I don’t think that Adrien’s childhood friend let him win when they played boules.

The way I understand things is that they would play a game of boules, Adrien would lose and instead of having to ask for a second game, a revanche, to be followed by a third one, a decider or a belle as it’s called in French, if Adrien won the second game, his friend would throw the cochonnet which means a new game is starting. In other words, Adrien’s friend took it upon himself to start a new game, thereby giving Adrien a chance to be the overall winner in the best of three games of boules that they could end up playing.

The linkage I see with what comes before in the text is that Adrien’s boss is willing to give him a chance to be involved in the company and part of the game.

I don’t think anyone let anyone win.

“Offrir sa revanche” only means allowing to other one to have a rematch. It’s merely an opportunity to beat him. It’s still kind of a gift, because if you win the first round you should have won, so it’s somewhat kind to offer your opponent another shot at winning.

No to your other point. The problem still holds, because grammatically il could refer to both. However with context, it’s pretty clear that it’s his old friend who let him have a rematch. Right now he’s offering his friend a job in his company, just like he offered him a rematch in their youth. It’s an opportunity.

Being said that I could well be over-reading :

Les points de suspension avec lesquels se termine le paragraphe me laissent penser à une charge implicite.

De fait… celui qui, aux boules, lance le cochonnet, choisit le terrain.
Et, s’il cherche, à tout prix, à gagner il va choisir son terrain.
C’est encore à dire, qu’il lance le jeu sur une configuration de terrain qui lui est avantageuse.

Donc, et même si je m’accorderai avec les autres contributeurs pour dire que l’ami d’Adrien ne le laisse pas gagner… on réalise tout de même que tout va dépendre du terrain choisi au lancé du cochonnet… car… si (et c’est je crois pouvoir dire le cas) il choisit le terrain le plus favorable à Adrien… sans chercher à perdre, il lui donne tout de même… une chance de plus.

En appui de ma lecture, je signale un mémoire de Céline Cachat sur Aurélien (l’oeuvre d’Aragon dont il est ici question) intitulé le kaléidoscope et le mentir-vrai.

Mentir-vrai ? Allez! Je… vous laisse votre chance… !

The ellipsis ending the sentence you quoted makes me think of some implicit charge.

As a matter of fact, the one who throws the cochonnet chooses the field and if he wants to win, he will choose the field being the more advantageous to himself.

So, and even if I will agree with the other contributors to say that the friend of Adrien does not let him win …
we understand that everything will depend on the chosen field, because … if (and I think I can say it’s the case) he chooses the most favorable field for Adrien … without trying to lose, he nevertheless spontaneously offers some significant advantage to him.

The title of Cachat’s memoir is important to catch in that sense. Word for word it renders something like to lie truely… would this be what USians call… alternative facts ?