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).
To understand how these interact, you must understand how these abilities work.
Vital Strike
Vital Strike is a feat that allows you to double the weapon's base damage dice when performing an attack action (standard action).
Spellstrike
Spellstrike is an ability that allows you to add a spell effect to a weapon attack through two methods:
- While holding the charge of a touch spell, you can deliver the spell on your next successful weapon attack.
- Instead of performing the melee touch attack you can normally do as a free action immediately after casting a touch spell (see the text below from the combat rules), you can perform a melee weapon attack in order to deliver the spell.
Touch Spells in Combat: Many spells have a range of touch. To use these spells, you cast the spell and then touch the subject. In the
same round that you cast the spell, you may also touch (or attempt to
touch) as a free action. You may take your move before casting the
spell, after touching the target, or between casting the spell and
touching the target. You can automatically touch one friend or use the
spell on yourself, but to touch an opponent, you must succeed on an
attack roll.
Note that spellstrike is not an action itself. It's a special ability that triggers when (#1) you perform a weapon attack while holding a charge or (#2) cast a touch spell.
How do they interact?
Vital Strike does NOT affect spell damage. The feat explicitly only affects the weapon's base damage dice and does not interact bonus damage from any other special ability.
You can use Vital Strike with spellstrike method #1. While holding the charge of a touch spell, you can perform an attack action and apply both Vital Strike and the spell effect on the attack.
You CANNOT use Vital Strike with spellstrike method #2. The attack action always requires a standard action, and you cannot substitute the free melee touch attack from casting a touch spell with an attack action. This means you will not be able to cast a standard action spell and Vital Strike on the same turn.
Best Answer
Yes
The Magical Trickster feat (Rogue 4) allows this:
Outside of that feat, there is no other method I am aware of at this time.