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

AND

You must wield a finesse weapon OR

You must wield a ranged weapon

AND

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).