European-style combat would need some changes. Plate armour with this low-friction surface would be of limited value. Full sets of steel plate were extremely difficult to breach; typically, in hand combat, an attacker would have two options. First, they could be conservative with committing their weapon, using footwork, finesse, and constant probing attacks until their opponent made a mistake and revealed a chink in their armour. Your low-friction finish wouldn't help much against this sort of thing, as the only significant attacks would be against unarmored spots or the weapon. Skilled opponents might not even notice.
Secondly, they could use the enormous mass of their weapon, aimed at spots from which their weapon could not deflect, to shove their armoured opponent around until a gap was revealed in their opponent's armour. Since this suit doesn't give your adventurer more coverage, and will only moderately affect mass weapons that aren't really intended to pierce plate armour anyways, your adventurer will only gain a moderate benefit by deflecting a certain small percentage of blows that might otherwise have bitten in and gone through the plate. Some opponents would be greatly surprised, but most would probably find the difference noticeable, but ineffective.
For this sort of combat, having your shield's surface be near-frictionless would be amazing. Many tactics used against "sword and board" opponents involve trying to shove the shield aside somehow, forcing them to commit large amounts of momentum to their weapon on questionable shots. But a skilled shield user often controls the angle of contact between weapon and shield, and most often the shield user's objective is deflection, not absorption. Deflected weapons in these circumstances will really drag the wielder off balance, and a full-bodied shield press will be almost impossible. For my money, just coat the shield, make yourself a medieval Captain America.
For point weapons like lances, pikes, longbow arrows and such, this sort of surface would also be extremely effective. Low-friction surfaces would deflect these almost effortlessly. Luck, as in finding the visor slit etc., would likely be the only way these weapons would do any damage.
All of this is predicated on steel plate. Using iron would present a number of issues that the low-friction surface would likely not mitigate. If you have not already, you should read up on iron vs steel for armour before committing to iron for your adventurer.