Usually these "unsuspecting" fiances are not nearly as unsuspecting as we may assume.
In fact, it may have been part of the reason of the ring came out at this time.
My point is, go ahead and worry if you want to yourself. For your friend. For the groom. For yourself. For whoever.
But one thing to keep in mind, people usually keep some little bits to themselves that they don't tell even to friends.
And even when they don't, things work out in the end. Somehow, as they have chosen.
Me, in that situation, would air my opinion once if asked by my friend. But after that, once the decision is made and it's said and done, it's about support.
Mantra for when you wanna yap: I'm supporting my friend.
It doesn't necessarily mean I support all her decisions.