Here's a list of five things you should NEVER say to a friend who just got engaged . . .
1. "That isn't the ring you wanted!" If her fiancé picked out the ring, it might not be the EXACT one she had wanted . . . but it's still special, so you need to be respectful of that. Focus on the fact that she got engaged, not the actual ring.
2. "He should have said/done [blank] when he proposed." The proposal is a special moment between your friend and her fiancé, so if SHE'S happy about it, it's really inappropriate to judge or criticize.
3. "It's about time!" The issue here is that it's implying your friend is late to the party, and it almost makes the engagement feel less special since they've been together for so long.
4. "I'm sad I'm single." It's hard when you're single, but a friend's engagement isn't about YOU. Be happy for HER.
5. "Are you sure you want to do this?" She said "yes", so, yeah, she's sure. If you truly have an issue with your friend's motives or decision, pick a day a few weeks away and talk to her then, after things have calmed down.