You're parsing the sentence incorrectly. It's not
[随分仲]が[良い], but rather
[随分][仲が良い]. That's why you can't find 随分仲 in any dictionary - it's not a word.
You are correct that 随分 means "extremely" (well, I might weaken that a little - it's more like "quite" or "very"); here, it functions as an adverb (not an adjective) that modifies 仲が良い. If you consult any dictionary, you will find 仲が良い, which is a set phrase. See, for example, Weblio EJ-JE, which gives "close; intimate; on good terms" as a definition.
With this in mind, we can now look at the sentence you gave: 彼氏と随分仲が良い. One possible translation for this is "[someone] is on very good terms with [her] boyfriend." Since 随分 is just an adverb and not part of the set phrase 仲が良い, the sentence would still be grammatical if we were to remove it, leaving just 彼氏と仲が良い, which we could translate as "[someone] is on good terms with [her] boyfriend." Context will dictate who exactly "[someone]" is supposed to be.