Whatever they ask about the price fluctuation, you should not have an accurate answer for it.
Be on the safe side to avoid being blamed later on.
Tell them your history maybe. If you are still holding your bitcoin up until now then it should be enough reason to give them a positive reaction about it.
We don't know who manipulates it, we don't know if the market really reacts whenever they sell, we don't know how much amount they are holding to make the market move or not. No one knows.
You can recommend it but always tell them it's not your fault to whatever will happen in the future. They should still be responsible for what they are entering.