Agree with the others that the best bet is to tell them you're sorry and let them know you're there to listen if they need it.
Something that can help the dad, especially, is to be willing to *not* talk if he doesn't want to. Take him to the lumber store or computer shop or out to coffee, talk about whatever he wants. The mom generally gets 99% of the help and sympathy, dads are left out. Also, dads don't always feel the loss as much or as deelply as the mom does, at least at first. I'm basing that on how my husband felt when we lost our babies, how my dad felt when their last one was stillborn, how my father-in-law felt when his only (biological) child was lost to miscarriage and that was the only chance they had.
My husband felt bad, but mainly felt bad for me. The loss of the babies didn't quite feel as real until long afterwards. The female relatives were all concerned about me (well, for the first 2 miscarriages) and the male relatives weren't sure what to say so they didn't say much of anything. He had no support other than what I gave him.. that wasn't as much as I wish it had been because I was more needy than giving at that point.
Don't push about the subject - they may or may not want to talk now, in a month, in 6 months - or ever. Follow their lead and you will be a very good friend. You must be a good friend anyway, just by asking for advice you are showing you care.
Sympathy to your friends and it will get better in time, I just won't presume to tell them how much time it will take to heal emotionally. It's different for each family, each loss.
mom to 7 living children and 7 angels
Sadly what you posted is all too true. I was too depressed at the time to realize what my husband was going thru. I didnt think he was going thru anything because he was just himself and taking care of me. Only later have I realized what you posted was true. Thanks for a good post.