A really, really close friend of mine's Mother just passed away.
They are from Czech Republic.

Normally I'd send flowers to my friend, but I wanted to check if there is a more cultural kindness "gift" I should be sending.

I do not have an opportunity to be with them during this time.

Are they living here?

How would, should the circumstances be reversed, you feel if your friend sent you some flowers?

By all means reach out to your feeind.

I think cards are more common but I doubt someone would get offended if you throw some flowers with it unless they have to pick them up at the post office, that could be an unnecessary hassle (I don't know how delivery works).

I think it is better to send money, because a funeral is not cheap and the family
have to organise their "new" life without the person , who passed away.

I think money is risky option, many cultures do not see that as a gift but rather a handout or a comment on their ability to manage their affairs.

I think flowers is fine, particularly when a message of sympathy and support is included; even if it is not culturally matching to the occasion, flowers are appreciated as a gift here, and would be taken in the spirit intended.

Sorry, but I think this is a big no, except when you really know that this is a tradition.
I would be horrified about that thought, if I haven't learned on my grandma's funeral that this is also a tradition in a small corner of my own country (Slovenia). Relatives were putting envelopes with cash into my father's cloak.

The loveliest gift we were given was a candle in a beautiful holder, and burning candles when mourning is quite international.