Balancing an Enchanted weapon

I’m a relatively new Dungeon Master and am currently running my first campaign with a couple of friends. I am asking about some advice for balancing a home-brewed weapon.

The player is a silver dragonborn Fighter, I gave them a standard long sword with a dwarvern mechanism that can create a 15ft cone of cold that was the same stats as the player’s breath weapon, including the saving throw and scaling of the damage, essentially giving them the ability to use their breath weapon twice per long rest.

The weapon itself is not enchanted so he PM’d me asking if he could get the blade enchanted.

So I came up with this:

This weapon has two charges, that recharge every long rest.

This requires attunement.

The chilling blow ability requires the con saving throw against the user’s intelligence spell cast DC (8 + Proficiency bonus + Intelligence modifier). On a successful save the target takes 2d8 cold damage, on a fail the target takes half.

Players have not been in combat for a while for some recent quests and next session they will need to defend from multiple waves of soldiers that gets progressively harder.

In conclusion is the weapon balanced, and if it isn’t is it justified to give them it anyway so they have an increased chance of survival with this next encounter?