
You don't know how their relationship work and you also don't know these people anymore. You can't judge someone you dated from 10 years ago and apply to to their current relationship. It's RUDE. See this in this POV, just because you treated someone shit in the past 10 years ago, your ex shouldn't try to act nosy and stir up drama in your current relationship.

Based on how he was treating his ex, you can easily tell that he hasn’t changed. Even Shinwoo noticed that. Shinwoo doesn’t even feel jealous of the ex, he feels bad for him because he’s been in that same exact position.
This is not “acting nosy” and “stirring up drama”, this is looking out for someone else before they get hurt the same way that you did.

Based on. The ex has no business because he no longer knows these people. Han is also trying to change and whether he's correct or not, it doesn't matter because he's giving MC more anxiety despite not having how their relationship work and he just met them. That's being a nosy. If a stranger start speaking on my relationship like that, I would despise them. What grandpa said comes from a pace of good heart because he actually knows Han really well while he ex dated him for only 2 months 8-10 years ago. Don't you see how ridiculous it is to talk shit about your ex from years like that to their current partner?
He's not adding anything or helping MC, just giving him more anxiety and make him more miserable so what's the point here? MC knows HAN better than him so he should back off

Q10 years ago Han was in hugh school. It takes at least 3 to 4 years to graduate from high school and enter College and I don’t think he started dating that ex of his immediately after getting into uni. So 5 years is a fair assumption. And the ex isn't even hating on Han, he is just warning Shinwoo. If I was in Shinwoo's place I would have wanted to be warned as well

He is helping MC tho? He is telling him to call shots and not give away too much of himself which are both valid advices even if your boyfriend isn’t as fickle as Han. Better be anxious than get hurt no? And again this was definitely not 10 years ago since they met in uni and at that time Han was in high school with Shinwoo. Telling someone to be wary is not talking shit about their relationship

He was in college 8 years ago when he saw shinwoo again. It's implied that he got Jealous and started dating other people. The car crash happened when he was already working abroad which was 5 years ago. And no, 5 years or 8 years doesn't make it better. The ex just met shinwoo and has no business talking to him like that.
I mean, if my ex was anything like Han used to be (or still is, based on how he treated his ex), then I’d personally warn the next person too.
I’m not the type to just let someone walk into a hell without at least letting them know what they’re in for.