A friend keeps getting messages promising to double their Malairte. What do I tell them?

Tell them directly: nobody legitimate ever doubles your coins. That is the single most reliable rule in cryptocurrency. Any message offering to multiply funds in exchange for sending some first is a scam, every time, without exception. Walk them through the specific message together. Point out the urgency, the impersonation of a known person or project, and the address they were asked to send to. Suggest they block the sender and never reply, because replies confirm the account is active and invite more attempts. Encourage them to ask you before acting on any unsolicited crypto message for the next few months. Most scam victims report later that they sensed something was wrong but had nobody to check with.