For reference we all work together at a chain restaurant. Sam[29M] and Tori[23F] moved to our town due to Sam's other job in the military and to save for their upcoming wedding. Sam and Tori have been engaged for about a year and are planning on getting married within the next few months. I got married last year so Sam has asked me for wedding planning advice and talks to me often about getting married and his plans around that, so we've become close over the months that they've worked with me.
A new guy named Carter[23M] started working with us about a month after Tori and Sam started and ever since Tori talks and flirts with Carter whenever Sam isn't working or occupied. I've caught them in the break room numerous times being way too close with Tori basically all over him. Carter happens to live on the same military base as Tori and Sam, so Tori has invited Carter over to her house specifically when Sam isn't around.
I keep overhearing Tori talking to Carter about how she wants to leave Sam and how much she can't stand him. All Sam talks about is how much he loves Tori and can't wait to marry her. I'm not sure if Tori and Carters relationship has extended beyond an emotional one but my best guess is yes. I don't want Sam left in the dark and suddenly dumped so close to their wedding and have gone through such a hard thing with me having known the entire time. I almost feel like I'm lying to him when I'm helping him plan his wedding knowing Tori is openly telling another man her plans to leave him. How do I tell Sam without this blowing up in my face? I want to make this situation the easiest for everyone as Tori clearly wants a way out and Sam deserves someone who wants to marry him.