“It’ll never last.”

Even if you really think the person you're talking to is going to be broken up with, don't say this. It's an instant way to make someone get upset at you and might even make them want to stop being your friend.

“He/she is probably cheating on you.”

Being in a long-distance relationship, it's hard enough to go off of each other's schedules, make sure you are still enjoying yourself, etc. So if the person in the LDR thought their S.O would cheat on them, they more than likely wouldn't be with them.

“How do you expect to see them?”

Sometimes this is asked and taken the wrong way. Instead of saying "how do you expect ___?" say, "when will you see them next?" or "When do they get to come home?" that's taken a lot better and sounds a lot more positive.

“They’re probably lying to you.”

As I mentioned before, it's hard enough maintaining yourself while your S.O is away. If you're constantly a Debbie downer or telling someone who is in an LDR that you have very little faith in them and their relationship, I can promise you they won't want to keep you around.

“How do you know they're not embarrassed of you or that his/her friends even know about you?”

I guess it'd be kinda hard to find out if someone's S.O didn't tell their friends about them. However, if you're supposed to be that person's friend and you're asking them stuff like this, I can almost promise you they are going to be embarrassed to talk about you to their S.O.

If they legitimately think their S.O is hiding them then yes they might ask them, but personally, if I had someone in my ear telling me that my boyfriend was embarrassed by me, you bet your bottom dollar I wouldn't be talking about that friend.