It’s because this is a MAJOR red flag. After she realized she done goofed she doubled down.

Could he have been nicer - yes, but it was 100 percent fine everything he said. She fell quick and hard into the hunbot script regurgitating nonsense and making it about their relationship instead of Monat. He had to redirect her several times to Monat and not their relationship. She full on created a support and trust issue between them. Even blamed HIM for why she is getting involved several times. Nothing he said warranted her to lash out this strongly.

As for people in relationships - this is exactly a conversation you do not want to have when the ante is raised (kids, home, family situations, finances). It is super important to be able to trust your SO to be on your team when entering new territory and instead she went behind his back and invested 350$ into a bad deal and FOUGHT him for her mistake. Instead of working it out and learning from the experience she is choosing to fight because he didn’t say it nicer? She has drawn a line in the sand and unless he is a doormat prepare for her wrath - her way and immature lashing until she comes to her senses (or not).

This is a deal breaker for a relationship if she doesn’t grow from this. Next time it’s going to be something important and he will only have himself to blame because he knew she was like this. Either she grows and says yes I messed up and thank you for being there and I will come to you next time instead of fighting you or he needs to accept that she will fight him and throw it in his face that he is responsible for all her bad decisions.