What to do if a player expected much more than DM gave?

I was not the DM in this session, it was my GF, but she does not speak English so I’m here to ask for all of us. We’ve faced an issue that seems we can’t solve.

It was our 2nd game, no one of us ever played before. We played Lost Mine of Phandelver and finally entered a room with a hostage in a cave. Goblins told that they don’t want to fight and proposed a negotiation. We had a jade statuette and one of the PCs told the goblins that we have it (The DM said before that because it cost a lot, the goblins greedily want it).

After that, the goblins asked 20 gold coins and the statuette in exchange for a hostage. Since they had an advantage (3 PCs vs 6 goblins) I told the other players that this is a good price, despite is having to give out almost all our gold, but my friend decided to intimidate them, roleplayed and rolled a natural 20. The DM said that the goblins seemed kinda worried, but since they had a huge advantage, they (only) reduced the price to just the statuette.

After that, my friend was visibly disappointed (and still is) almost to the point where he refuses to play anymore. We were tired at this point since it was our 6th hour or so, and I can understand his mood here.

When we discussed this situation after the game, he explained his point: he had rolled a 20 and this means (in his opinion) that goblins shod have released the hostage without demanding any price because it was a 20. I tried to explain to him that it is up to DM to decide the consequences of any actions players do, and in this situation, the DM decided not to give us more. But the player still thinks that the DM was not right in this situation and a 20 is a 20 and he is ready to even have arguments for this game situation. I tried to convince him that the goblins had a huge advantage here and it was not possible that they give us the hostage without any price, but it seems impossible to convince him. He is negative because of the fact that the DM is the final judge and it is the DM’s responsibility to decide what happens next.

We’ve tried to find some solution on what to do in situations like this and did not come to anything.

What can we do?