First of all, I'm very sorry for your loss. It sounds like a difficult situation. Regarding your question, yes, money memorials are still common here and appreciated. It varies by region of the country I learned recently. I lost my mom last month and one of her cousins died in CA in May. I live near you and it is very common for the family of the deceased to receive money. My great aunt in CA said the only people who sent them money were her MN relatives. However, she found it touching and really appreciated it.
If you've already given a plant you don't need to also give money, but if you can afford it and if you were close to your friend it would be a nice gesture and would be appreciated. Contact the funeral home handling the arrangements as to who to make the check out to. I often write on the check "family wishes." We also received one check for my mom made out to "cash." The check should usually be made out to whomever is planning the services, whether that's her parents, fiance or children if they are old enough.
My parents had many friends and business associates and my mom was only 70. We received thousands of dollars in memorials. My dad didn't need the money for the funeral/burial or his own purposes so we are establishing some sort of permanent memorial for my mom at my parents' church and we are helping out several charities. She died of complications of scleroderma, a little known auto-immune disease that doesn't get much research money, so we are sending them a check. We donated to the beautiful hospice facility where she spent her final 24 hours and we donated to local food shelves in her name to feed the hungry for Thanksgiving. That made my dad very happy to be able to do that. I once gave a large check to a family that lost their dad/husband unexpectedly and I assumed they needed it for funeral and living expenses, which is what I had intended. So, every situation can be a little different.
Best wishes.