Anyone who has ever been in charge of anything has offered a spot to someone who turned it down, either right on the spot, or later. Being aced by a better offer comes with the territory. If I was the one who had offered you a spot, a later collaboration would only confirm my first decision that you had been someone I wanted in my group.
So, what should you expect? I certainly wouldn't bring up the past in any negative way, I'd say I was glad to be working with you.
And what should you say?
(1) Don't apologize. I didn't do anything for you, and you didn't take anything from me, so you don't owe me anything. Even if you cancelled so late that the position stayed unfilled for a while, that's not on you. If there's something of mine that will fail without staff that's on me. It up to me to have a contingency plan, and I will. "Many's the slip, 'twixt the cup and the lip". Anyone who belongs in their job knows the difference between someone on-board and someone who made nice noises about an offer.
(2) Don't ever bring up not taking the spot. I knew at the time you either got a better offer or had a family crisis, so now I know you got a better offer. If I want to know why you took the other offer I'll figure it out from working with you.
(3) if this person does bring the past up, or complains about you taking a better offer, or whatever, don't respond with anything about better or worse. Make it a matter of personal preference, like a food choice, say there was something about the other offer that really seemed like it was a good match for you. If the other person asks again or for details, deal with them like someone who has had too much alcohol and is saying foolish things. Smile, repeat your first answer patiently, smile again, and then talk about an clearly unrelated subject.
I don't know if this is great advice for people playing academic politics. I didn't earn my ph.d. as a grad student getting stipends, I was working full time in the r&d end of my field I had a wife & children and I had tangential research that interested me to pursue and write up for a thesis and a dissertation and some projects, and I took graduate level courses classes that interested me, and I paid tuition and fees. That wasn't as unusual then as now.