Try reaching out to your Spirit Guide and ask them to contact your friend's Guide to see what help, if any is needed, that you can give. Just because you do not like where they might be headed, doesn't mean that they are on the wrong path. To me the only time for direct intervention would be to stop self harm (death, spiritual damage).
Send prayers out of protection and guidance for your friend. Give them true love. Sometimes it hard to see other's "falling or straying" from the path, but look to your path first so you too don't get lost.