Here's my view (and the solution below the heading):
To what extent is editing answers allowable?
Not much! We have to remember that it's their answer, so we have to be sure that they wouldn't (or, objectively, couldn't) disagree with any changes we make.
Fix a broken link? Yes. Improve ambiguous or poor grammar, when we're sure we know what it should say? Yes. Copy (without any commentary) factual parts from an existing link in an answer? A conditional yes, because they have included the link anyway, but only if the answer doesn't include any part of the linked material already - because the original author might not agree with everything in a link they give, and they might have already included the only part they agree with. Therefore doing this is risky.
But... Add anything subjective (commentary, analysis etc.)? No, because we don't know the author would agree with something subjective that we might write. (I know they could rollback an edit, but we don't know if or when they would see an edit, to then decide "no, I don't agree with "my" answer saying that".)
There is an answer to a question that is almost a link-only answer
(See later update ** below) I've seen the answer and currently it is IMHO a candidate for being deleted as "not an answer" (NAA) (see here for the relevant Meta article explaining that link-only answers are NAA).
but it is a pretty good link.
(See later update ** below) Agreed, and that is why, if someone flagged that answer as NAA and I saw the flag, I would convert that "answer" into a comment (for the reason explained here on the use of comments - see point 3 under "What are comments for"). The current lack of explanation of how that link answers the question, lack of summary of relevant info from the link, or analysis in pointing out the most important points etc. - all the things which make a good answer, good - mean that as soon as that link "dies", the answer is unfortunately almost useless.
[** Update: The explanation above applied when this Meta answer was first written, and would apply to similar situations of link-only answers in the future. However the original version of the specific answer on the main site being discussed here was subsequently updated and much improved, after this Meta question was first asked, and it is now definitely not a candidate for deletion as "not an answer" (NAA).]
Write your own answer!
That solution to this situation is suggested in this answer on a relevant Meta question.
Include a link to the existing answer and credit them for providing the external link originally (you must also include the external link in your own answer, in case the original answer with that external link is ever deleted - you can't rely on it "staying around").
Then write your own answer with whatever additional explanation, summary, commentary etc. you were previously thinking of editing into their answer.
That decouples you from needing to wait for, or otherwise influence, the original answer's author to agree to add your edit to their answer. You can write your answer at your own pace. It also keeps ownership (and therefore responsibility for the contents) clearly separate between the two answers (yours and theirs).