I have two online friends, K and D. K lives in England, D lives in Italy. They met in a Discord server that I now run for LGBT+ people (although K is male and D is female), and after a while we found out they had decided they were together: it was amazing and everyone was happy for them! A few weeks ago, they met up in England for a week and had a great time together: the pics and smiles proved it, and it looked like they really loved each other.
Today, I've turned into the third party for both of them as they confessed some things to me that have made me very worried. I think K is being possessive because of his previous relationships ending in catastrophe/betrayal. I got a message from him asking me to tell D he loved her in all small letters: he always writes in proper grammar, so I was immediately alarmed. He asked if I was his friend, and I said yes but asked him what happened. I told D, and she said to tell him she loved him too, so I did. He said "it doesn't feel like it"; "i feel like she hates me". I'm going to directly quote him now:
"I just feel like I am so insignificant to her. She never makes time for me. I'm always her last choice. I am not with her. So her friends are more important to her than I am. She never has time for me. She just goes out every night until 2am with her friends."
This is untrue. I asked D about it, and she told me that every time she sees her friends, he gets upset. When she told him she only has her friends in Italy irl, he "got mad". Apparently he gets upset every time she tries to have fun with her friends: if he was there she would have much more time for him: and they still text and voice-chat each other every day. She hasn't been cheating or anything, and she told me she loves him but doesn't know what to do, since she feels like he doesn't trust her anymore. We don't think he has a therapist (he used to).
I think it's possessiveness because of his old relationships but I just don't know what I can do. I don't want to say anything to either of them (especially K) at this point because I'm afraid I'll screw things up. I need help: I'm dealing with other problems of other people and my own problems too, but this is the one I really have no idea what to do next on. Please help..
EDIT: D tells me the situation is getting better, so this is no longer as urgent: but with everything she told me I'm 100% sure the same thing will happen again and am still very worried