Instead of focusing on the assurance just to convince them to invest in bitcoin, tell them the risk factors of it and why it's worthy of taking a risk. There is nothing to be afraid of when they ask you about the questions related to the crashing of the price because that's normal and could happen in bitcoin. If they are really interested in bitcoin, this kind of factor won't discourage them easily. In fact, it will help them to be more careful and critical in investing in Bitcoin. Nothing is a 100% guarantee or assurance in investment, so why panic when it is natural in the field of investing.