Good question... I've been put into similar situations, but it was more of a feeling and I guess I just never said anything for in fear if I did that it would come true. Now I beat myself up over it because I feel like I should have said something and think that maybe if I did they would still be alive. So because of past regrets yes I would tell the person and try to do everything in my power to make sure the person was going to be safe and no harm would come to them.
That is a very tricky question.
The problem is if you do tell that person and it still happens they will hold a grudge toward you. Why? Because they will think you did something to influence that. Than it will escalate to telling other people about it. Not a good idea.
Than if you do tell someone and they do prevent the bad thing than not only are you messing with fate the other person will keep coming to you if they have trouble. If you do not see anything bad but the next day they are hurt again we get the blame. It is a never ending process.
What you can do is instead of telling the person everything drop a hint and leave it at that. They person has to figure it out if not than you did help but the person just did not get it.
I am not telling you to hold information or tell them everything just be cautious about and think things through for your own safety as well your family.
Fate is not set in stone, we may always choose what to make of our future. There is no divine retribution to fear from acting on your knowledge, the only consequences you may worry about are the ones in this world. Whether or not you wish to change the future is your choice, though you must be mindful of what choices your friends makes as well. If they know about magic I would suggest telling them what you know, if not then perhaps a hint would be best. Simply give them a piece of advice that may help them, but leave the decision of their future to them. Our relationship to fate is not that of a raft floating in the ocean, but of fish who's swimming drives the currents.