Paladin Oaths are meant to be taken seriously
The choice of oath is a significant part of the paladin class and it is meant to be a serious choice. The PHB suggests that even before reaching third level and choosing the oath as a class feature, you should probably have read ahead and thought about which choice to take depending on the nature of your paladin. Furthermore, the sidebar BREAKING YOUR OATH states that:
A paladin tries to hold to the highest standards of conduct, but even the most virtuous paladin is fallible. [...]
A paladin who has broken a vow typically seeks absolution from a cleric who shares his or her faith or from another paladin of the same order. [...]
If a paladin willfully violates his or her oath and shows no sign of repentance, the consequences can be more serious. At the DM’s discretion, an impenitent paladin might be forced to abandon this class and adopt another, or perhaps to take the Oathbreaker paladin option that appears in the Dungeon Master’s Guide.
If your friend's paladin chooses an oath but disregards the oath's tenets and has no intention of adhering to them, I think that would be fairly described as wilfully, unrepetantly violating the oath, and so that paladin's continued paladinhood is called into question - the DM can force them to change class based on their behaviour.
This is a quite watered-down version of rules from prior editions - 3e strictly stated that a Paladin who transgresses against the class's vows loses their paladin abilities and must atone in order to regain their powers. 5e is less strict about this, placing the matter at the DM's discretion; however, it is still clearly intended that the Paladin is meant to stick to the vows of their oath.
If your DM and your table don't care about the tenets of the various oaths, that's entirely your business. If your group doesn't find worrying about paladin oaths to be any fun and you'd rather just keep the mechanics and abilities but play differently, that's fine, and that's a valid way to play.
However, the default assumption is that oaths are important, and people on the internet who don't know your group can't tell you whether your friends think differently! If I were a paladin player taking my oath seriously, I would be very annoyed if other paladins got to break their vows with impunity, and if I were a player ignoring my oath I would not enjoy the DM unexpectedly telling me that I needed to change my character.
Ultimately, the discretion lies with the DM, and your friend needs to talk to them about whether or not they can play this way. Ideally they need to make sure that everyone playing is on the same page about this, especially if they're all playing paladins.