Ultimately the link itself isn't the problem; it's the answer extrinsic of the link. If the text that appears in the answer is sufficient to be a helpful answer to the question, then it's a helpful answer; if the text is not sufficient to be a helpful answer, and only following the link gives you a helpful answer, then it's not one. Focusing on the presence or absence of a link itself is a mistake. I'd simply suggest the user evaluate his/her answer in the context of the text available in the answer itself, and then move on if the user disagrees with you (both because it's not worth your time to argue about this substantially - that's what voting and such is for - and because you might not be correct in all cases.)
