Is there a word meaning "well-intentioned but harmful"

Solution 1:

You almost have it in your question - ignorant advice. I think uninformed comes close but is rather just factual in tone, it doesn't quite cover the good intentions and the dire results.

One word that is often used to convey both these aspects, and the accompanying emotion, is

ill-advised

"Their friend's ill-advised suggestion being followed led to dire consequences.

CED: 1. not wise, and likely to cause problems in the future: 2. unwise or not carefully considered.

Solution 2:

A single word doesn't come to mind--but "the road to hell is paved with good intentions" tends to get the point across.

Solution 3:

How about uninformed? From Lexico:

uninformed: Not having or showing awareness or understanding of the facts.

If someone makes an uninformed suggestion, one may assume the suggestion is well-intentioned. After all, it is a suggestion. Also, by virtue of being uninformed, it's likely that the suggestion, if implemented, will lead to undesirable results, with how undesirable depending on the circumstances. After all, the suggestion does not reflect an understanding of the relevant facts.

Your examples:

"Their friend's suggestion was uninformed" or "Their friend's uninformed suggestion" or even "Their uninformed friend's suggestion"