I don’t know either of these people, but I am certain the choice to leave the relationship was not literally only about the incident described.
All we have to go on are a few words from someone not even in the relationship, so all we can do is project our own ideas and feelings from our own experiences and imaginations into the situation.
If that’s what you’re asking about, then in my case, apart from mainly being massively certain that the situation is at least 10,000 times more complicated and not really about this incident, I would tend to project and imagine that:
* She may not have liked “LUV” as a form of “love”.
* She may also have felt that if someone loves someone clearly, they might tend to want to use the clearest form to express it rather than a bent version.
* She may have felt the correction was a playful, loving, and/or fun tease.
* Or who knows what she was thinking/intending – certainly not me, and probably not him.
* He probably wasn’t actually clear about his feelings about her and the relationship, as hinted both by his perhaps-subconscious use of the corrupted spelling “LUV”, and certainly by his change of commitment to the proposal reportedly merely due to this minor occasion.
* He may have had other indications that there were characteristics of the relationship and/or her behavior that would not work out for him and/or the relationship, and this just act just resonated in his feelings (which are no doubt rooted in his own psychological material), and so he became suddenly aware of that, and so his perspective shifted.
* In any case, if this event results in a change in his thinking about wanting to propose marriage, then he was ultimately correct to at least not want to get married. (Whether or not it also warranted leaving the relationship, again, I have no information with which to even speculate.)