No matter what the item was, and no matter how much it cost, it was an accident and you need to send the message to your son that "things" don't matter by keeping cool aside from saying you wish it hadn't broken. Maybe you could send the message things aren't expendable by waiting a while to replace it, but then again, your son didn't break it, so that's debatable. He might learn to think twice before letting friends play with expensive things...for better or worse.
It's too late to try to extract and apology form the friend, and if he already apologized, you really can't do anything. If you can't afford a new one or feel you would like his mom to offer to pay for it-I think that's seriously iffy. It would be totally up to her to offer when she finds out.
If I found out my child broke something at someone's house I would IMMEDIATELY insist on replacing it, but I would think it was weird if they asked me to, especially if it was an accident. No harm in nicely letting her know it happened if she didn't find out.
If they weren't allowed to touch it, defied you and then broke it, that's a whole other matter, but if it just accidently got broken, that just happens someitmes.
I wouldn't do anything about it if it were me.