I noticed you used the word "meaning" and its accompanying adjectives six times in this post.
There is nothing inherently meaningful about a marriage or LTR, nor is there anything inherently meaningless about short term flings, casual sex, or serial dating. I've had flings and ONS that meant more to me than some LTRs, at least in different ways. You get what you give - usually.
I think you can derive "meaning" from either, though I think happiness is what we really should be extracting from our choices and pursuits.
And what you're describing about your friend feeling as though he "missed the boat," that's the human condition. The road less traveled. Had he gotten married at 25 and stayed faithful, he'd probably be wondering about all the women he missed out on in the prime of his adulthood. FOMO is what advertisers use to separate us from our money. It's powerful but futile; we have to live with the choices we make.
To quote Rollo, "I’ve never had meaningless sex; I meant to bang every woman I’ve ever banged."
The broader point is that whether they're looking for flings or marriage, putting the cart before the horse is a fool's errand, to mix a couple of cliches. If your friend, and BF's friend, really had such a miserable time slaying chicks all those years, perhaps there was something missing from within, rather than from the women and relationships they were pursuing.