Basically, one of my friends says that you can get Sneak Attack with any weapon if you are a Swashbuckler. The swashbuckler ability says that you can get sneak attack with any weapon attack as long as no other enemies are nearby. So, can he get sneak attack while using a non-finesse weapon as a swashbuckler?
Your friend is wrong
The Swashbuckler feature "Rakish Audacity" says:
You also gain an additional way to use your Sneak Attack; you don’t need advantage on the attack roll to use your Sneak Attack against a creature if you are within 5 feet of it, no other creatures are within 5 feet of you, and you don’t have disadvantage on the attack roll. All the other rules for Sneak Attack still apply to you.
Normally, you need to have advantage or have an ally within 5 feet of an enemy. This ability allows you to get Sneak Attack without either of those two occurring.
However, in the basic rules, Sneak Attack is explained as follows:
Beginning at 1st level, you know how to strike subtly and exploit a foe’s distraction. Once per turn, you can deal an extra 1d6 damage to one creature you hit with an attack if you have advantage on the attack roll. The attack must use a finesse or a ranged weapon.
So, while you would be eligible for sneak attack simply by being within 5 feet of an enemy, you would not get sneak attack because the criteria for sneak attack has not been met. For sneak attack as a Swashbuckler with the above ability:
You must have advantage OR
You must have an ally within 5 feet of the enemy OR
You must be adjacent to the enemy, and no other creature besides the enemy must be within 5 feet of you
You must wield a finesse weapon OR
You must wield a ranged weapon
You must not have disadvantage on the attack roll
The Swashbuckler ability modifies the top part, but not the bottom two parts. So, no sneak attack with a greatsword (or his fists, or a table leg as an improvised weapon).
The Unearthed Arcana rules are specifically pre-release rules meant to gather community feedback. If there's an argument about these, it is definitely up to the DM to decide on them.
That stated, they have now officially printed the Swashbuckler in Sword Coast Adventurer's Guide. This version drops that Toujours L'Audace ability and replaces it with Rakish Audacity.
The new wording is much clearer.
In addition, you don't need advantage on your attack roll to use your Sneak Attack if no creature other than your target is within 5 feet of you. All other rules for the Sneak Attack class feature still apply to you.
Hopefully that settles the discussion.