I think my friend might be lying to me. How can I approach this?

So, about a year ago, my friend 14M (D) messaged me 14M (J) a screenshot of a conversation between him and his crush 13M (S). They had only known each other for around a year. It was a pretty normal chat, nothing crazy, but then he deleted the message and the following texts, and said something like, “oh shit, I shouldn’t have sent that.” That felt really suspicious to me.

When I asked him why he deleted it, he told me that S had told him a secret she’d been keeping for two years but didn’t want him to share it with anyone. He said I was the first person she told, and that he was only telling me a tiny fraction of the secret — like 0.1% of it.

He was super defensive about it and kept telling me not to ask S directly, asking me to respect her privacy. He also asked me not to tell other people, including another friend.

What really felt off was that I’ve known S much longer than D has, so it didn’t make sense to me why she would tell this secret to someone she’d only known for a bit over a year, especially knowing D has a crush on her instead of telling me when I had known her since I was 6. Plus, when I asked D what the secret was about, he hesitated when giving the first letter, which made me even more suspicious.

We never talked about this secret again after that, and it’s been almost a year now.

Recently, I had a shower thought that maybe D was lying or exaggerating the whole thing because of how secretive and defensive he was.

I’m considering asking S something like:
“Hey, you don’t have to tell me what it is, but did you tell D a secret? He mentioned it to me but didn’t say what it was. I just want to check if it actually happened because some things seemed off. No pressure to share anything else, just a yes or no is totally fine!”

I want to know if D was truthful, not the secret itself.

What do you think? Is it a good idea to ask S like that? Or should I just let it go?

TL;DR: My friend (D) told me he was told a secret by his crush (S) that she’d kept for 2 years. He was super secretive, deleted messages, and wouldn’t let me ask her directly. I’ve known S way longer than him, and the story didn’t add up. It’s been almost a year with no update. I now suspect he might have been lying. I want to ask S if she actually told him a secret (without pressuring her to share the secret itself) to check if D was truthful. Should I ask or just let it go?

Thanks for any advice!