Grace, I love this aspect of learning a language and yours is a great example. We know the word for better as a comparative so the related verb's meaning becomes clear to us but then we express it in English - improves. Translating the meaning - brilliant! If Iwas allowed on this tablet I'd give you a lingot for re-enthusing me!
In that context , I'm familiar with "bests", but not "betters". Granted, the former has always been past tense, e.g. "She bested..."
If I heard "She betters her car," I would assume either she makes improvements to the car, such as the translation suggests, or more unlikely, that because she is who she is the car becomes better by being hers.
Jcih: where are you from, out of interest? These threads get well off the Spanish but are interesting. I have never heard this usage. I guess you are trying to apply it to current sentence (hence the rather odd scenario) but is there a more natural example you can give? I was thinking of two people in competition somehow but I can't hear it - I agree with the user who says "bests" him or perhaps "got the better off"?
I'm from New Zealand. This would probably be used more in a formal circumstance than an informal one. From my perspective "She betters herself" is using the word in the sense of the original sentence, but "She betters her mother" suggests that she is better than the mother in some way.
Indeed, it is interesting to observe just how differently we all craft words ;).
I tried that too, and it wasn't accepted, and while I still think it should be accepted, I can formulate an argument against it. In English (at least where I live in the US) if one says "She upgrades her car" it could be taken to mean that she is trading her car in for a better car, not that she is making her current car better, which (as I understand it) is what this sentence is supposed to mean.