Is there any practical way to get across to an acquaintance that his-or-her actions in regards to one relationship have really hurt older, mutual friends? I believe, for one, they feel discarded w/o merit, although they won't particularly show it.
My apologies for the delay in my response. My friend, nothing is as practical as just coming right out and saying what is on your mind. I caution you, however, if you are not a direct party in whatever transpired, it may not even be your place to say anything. Interjecting yourself in the situation only spreads the drama fire. The best course would be to let the principal parties handle it themselves. You may certainly express your support to the friends whom you think have been discarded. However, you cannot speak for others. They must do so for themselves. Believing your friends feel discarded is not enough. Confirm this with them first. You may be overestimating their state of mind, or even be entirely wrong about it. Also, from what I can gather by your question, these mutual friends are also secondary parties. Let the principles work out the situation for themselves. The best thing you can do is be available as a sounding board should either of them need one.