but like why?
just break off the engagement...
don't bother messing with her memories - i mean that's very invasive...
or at least explain it to her like a normal person...
if the person who poisoned the kid was planning on hurting the MC even after the engagement was broken, erasing her memory isn't going to change that, it'd just make her confused af... which she is right now kinda
I mean, they literally erased her memory just to avoid a bit of heartbreak/sadness which as a kid, is pretty eh standard (I mean you have to learn that at some point) and I mean they'd have probably parted amicably not like a divorce from a 10 yr marriage...
In conclusion, I probably shouldn't think too much when reading manga.