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)

There is no known glitch that does this. You didn't happen to record it, did you?

Sadly no. Both me and my friend aren't Undertale Speedrunner and we never exspect it to happen.
And the only thing he recorded was the Sans fight and after because he had problems with OBS.

The only thing you can see in the video is him getting his Health Bar turned Pink without loosing one HP.

So not really helpful in finding out how to get the Glitch.

The only thing I know is him dying in the first 5 to 10 Attacks and him one-shotting her at the second try and being immortale.

This sounds like a case of using cheat tools, not gonna lie.

Unfortunately, if you don't have a video of it, there's a saying in speedrunning for that: no vid, no did. Meaning that if you claim to have done something but don't have a video, then we only have your word for it and the latter part of the saying means you didn't do it.

Yeah I know, I had expected that no one would believe it without proof.

I was curious and wanted to know if it is a known Glitch or something that at least one other person has experienced.

But now I know, Thank you.

Update: My friend had a virus on his computer and needed to wipe it clean. He said he noticed that some of his other games were having performance issues or were not running at all. The Virus was on the Pc since sunday (24.07 or 07.24). I think the virus messed with the game data and made him accidentally immortal and Op.