Honestly its not a good advice to give on buying bitcoin at 18$ as some of them will blame you especially if you did this advice to your friends. Let them discover Bitcoin themselves and then if they want to invest you can help them but don't force them, not all of us have big patience and not all of us do long term investments.
For me ,you're right as if you make long term investment you should buy bitcoin at every price as long as you will have that iron patience to wait for the bull run.
They won't play the blame game if you explain them properly how the market works. They should be made aware of the "short term" risks involved when investing on bitcoin. They should know that the price might drop anytime without any signals or notice. They should be made aware of market manipulation so that they don't panic when the price starts to drop.
If your "friends" know that bitcoin is for long term investment, they won't blame you if the price drops on the short run.