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, we could take it out, 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.