Good for You on sticking to your guns. Of course you should not pay anything. You didn't arrange this, if STBX failed to plan that's his problem, if this friends were daft enough to let him mooch of them, that's their problem, and this incident will be a useful learning experience for them.
It's bizarre to me that the friend would think this was in any way reasonable or appropriate. After all, if STBX told them you'd pay, then the obvious arrangement wouldbe for him to collect rom you when he brought BabyMoss home, and for him to them repay his friend (I can't think why they'd think you'd be paying, but if you were, that would be the obvious method, to me!)
I think you handled yourself just fine, but in future, if he or any of STBX's mates try it, I would recommend that you avoid getting drawn into any kind of discussion about it.
As soon as they bring it up say"I don't understand why you're calling me. This has nothing to do with me, it is between you and Ex. You need to speak to Ex."
if they persevere, repeat "This has nothing to do with me. You need to speak to Ex" then end the call.
(If they claim he told them you would pay then the same answer works, too!)