How to determine whether a 10 days S.N. Goenka Vipassana course is safe for my friend or relative?
I would suggest that you trust yourself to make that decision for each individual you are contemplating the recommendation.

You have personally taken the course so you have firsthand knowledge of its contents and rigors. The people you are going to be talking with are 'close' by your description so you already know a lot about who they are and should understand (or be able to learn through discussion) approximately where each one is on their personal journey on the path toward Enlightenment - and what help they might need.

With those pieces of knowledge, and the additional understanding of the issues others have had with the course (references you provided in your question), you are in a good position to analyze the issues for each and make that decision.

For those you decide to approach, I would make sure to spend the time informing them not only about the benefits you obtained but also some about the potential negatives that others have had. In this way, they will have all of the information they will need to look within themselves and decide if this is right for them.

In my opinion, withholding knowledge of training which you feel could help another would be as bad as blithely recommending it without providing the understanding of the possible difficulties they might encounter.

Through using your judgement and taking the middle path, I believe you will have done your best to assist all of them on their journey.