It depends on what kind of Dreams they are having, If it's random and wierd than don't worry about it, but If they keep having the same dreams then I'd check into that...it's possible that it could be far more than a coincidence.
I think looking for signs is the worse form of vanity, thinking we have the ability to know God's mind more than our our other brothers and sisters in Christ. I think that someone thinking they are actual seeing signs is indicative of a real need for that person to learn the truly important trait of being humble, and not prideful. - "A wicked and adulterous generation seeketh after a sign; and there shall no sign be given unto it, but the sign of the prophet Jonas. And he left them, and departed." - New Testament, Matthew 16:4
God hasn't changed. Scripture mentions God giving dreams to people. And, He still does today. The dreams He gives will accurately fit with scripture if they are from God. And, I do believe that God gives dreams of warning also. If these dreams are from God that your friends are having, scripture will support the dream if they search it out to see.
The dream i had last night was.i held my pregnant cat up I'n the air to my face and said what a cut kitty kitty and she delivered a huge cat as big as a dog and it fell out on to the bed the end of dream. The next one was about the mail man he was delivering the mail and me and my husband were out side and he egnored us and kept delivering the mail .the end