OP, in my country you would send a floral arrangement with a note, not money, money at funerals are out of question. If anything, the deceased's family is expected to offer a meal after the funerals.

If you can't do this, send flowers or a card, and your friends live here or they'll be back soon, maybe cook something for them? Normally I would have called and asked if they needed anything and told them I'm there for them. I think grief is more personal than cultural, if you ask me. My Czech friend wanted to talk about her mom. We did it later on, we did it many times. I wouldn't be so worried, if you're unsure you don't have to do anything just right away, other than sending a card. IMHO.