Something strange happened to my friends' genocide route.

A little background: My friend has played Undertale in his life once and completed a neutral route 1 or 2 years ago.

Today he told me he's board and has no motivation to play the games he normally plays.

I asked him if he wanted to play Undertale and do the Genocide route because I knew he had never done either of the other two routes.

He agreed and we both started a whole new file. Me on another Switch user account and he on his PC. (Note: He has a new file because he had to wipe his computer clean 3-5 months ago.)

We both play the game normally until we got to Undyne the Undying Fight.

I got there first and beat her before he came to the fight. He had the same items and level as me.

He saw the fight for the first time and died in the first 5 to 10 attacks. I was exspecting to sit there with him for hours until he won.

But for some strange reasons on his second attempt, he one-shot Undyne as it was nothing.

At first I thought it was the game's plan even tho I never had seen this happen ever.

But after we started farming enemies in the Hotlands, he noticed that he no longer takes damage and hits every enemy he encounters with one shot.

This glitch lasted until the end of the game and in the sans fight his health bar turned purple but he didn't lose any HP.

I am 100% sure that he didn't change anything in the gamefiles or used a cheat.

So I want to ask if there is a glitch that can do this as well without changing game files or if other files on his computer like mods or plugins from other games might have change something and if this had happen to anyone else?

Thanks in advance.

(Sorry for my bad grammar, it's 3am for me currently)