“Can’t lose money if I don’t sell” argument with a friend

So my friend and I got into an argument about the stocks and investing. Originally we were arguing about whether putting all your money into a bank is a good idea but it then morphed into investments where her counter to anything I said about the volatility of stocks was “you cant lose money if I do not sell” or “the market will always go up” or “the government guarantees anyone who has less then 250k in their bank account”. Now, I did not like any of these arguments but I will admit I am a noob to investing, so I wanted to hear from other people regarding this on whether my friend was right or wrong. Also some sources would be helpful too!