As with so many words, both marriage and divorce can be either countable or uncountable depending on what you want them to mean in a particular context. For example, 'marriage' as a concept is uncountable, while 'marriage' as a period of union between two specific people is countable. For example: Is marriage an outdated institution? John had already had three failed marriages by the time he met Angela. Divorce is the same - it depends whether you are speaking in general or specific terms. In this sentence, I'd say the writer used 'divorce' as an abstract/uncountable noun because of the phrase 'ended in'. Common collocations with this expression are 'ended in disaster' or 'ended in defeat', so 'ended in divorce' sounds natural.

I'm thinking the apparent singular form of "divorce" in your sentence is used because the phrase "in divorce" works as an adverb. How did the marriage end? Abruptly. Amiably. In divorce. Basically, when nouns turn up in a sentence serving the function of an adverb or adjective, they don't take plural form. Notice how Su.Ki.'s other examples also work as adverbs (ie. describing "how", or the manner in which the action is/was performed).

countable and uncountable