Translation:Her father sells bento at the convenience store.
Actually, if you look at the main entry 'bento' in Wiktionary, you'll see that the English plural can be either 'bento' or 'bentos.' In other words, as is often the case for Japanese (or Chinese or Korean) words, the majority of which have unmarked / uninflected plurals, when those words are used in English, one can opt to use either the uninflected form of the original language or to add -s for a more typical English plural. Which form should you use? Academic use or use among those most familiar with the other language probably favors the uninflected forms, whereas references to everyday items are more subject to English style pluralization. A look in Wikipedia can provide examples of informed usage that seem to confirm these tendencies. Under the article headings of 'bento,' 'samurai,' and 'kanji,' the plural forms used are 'bentos,' 'samurai,' and 'kanji.' The English style plural is used only for bento, even though Wictionary shows both forms possible for all three.
I've usually learned かのじう as "girlfriend" so i find it a bit odd when it just translates to just "her." Same thing goes for かれ for "boyfriend" instead of just "he". Am I wrong to think this?
A meal is adressed as gohan, whereas a bento is a specific type of meal, stored and sold in neatly packaged and arranged boxes (aka. a bento-box). Although anything can be in a bento, it is still distinguished by the storage it is in. Not to mention that store-bought bentos are expensive, so they get adressed differently just by the respect towards more expensive things...