Should I keep this secret or not?

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We have friends, a couple but not a married couple. They have been together over 20 years. We don't see them often, maybe three times in the last 7 years. We do communicate via e-mail. The last time we met up with them we arranged to meet at a restaurant and before the restaurant we both talked about stopping at the same outlet store. When we got to the store the first person of the two I spotted was the male. He was standing near the women's dressing room. I 'assumed' he was standing there waiting for his girlfriend to come out so I walked by and didn't let on I saw him. A few minutes later I ran right into the female who had not been in the dressing room at all, she was at the opposite end of the store. None of the items she had in her hand were things that needed trying on. We walked together looking for 'him' and as we went by the spot where he had been standing I realized that from his vantage point he could see right into the dressing room...there is no door, but curtains on each cubicle. I put all this out of my mind but something she said in an e-mail the other day brought it into my head again. She said he has a great sense of humor. I've never witnesses his sense of humor at all so not sure who he shares it with. Wonder if he would think it was funny if I mentioned to his girlfriend that he had been peeping. I think keeping my mouth shut is probably the way to go as they both would probably have all kinds of excuses why he was standing there. Plus what proof do I have now months later? What would you do? Photo: pixabay.com/en/shopping-buying-buy-store-shop-1182902/ :