What do you mean? There are mutual funds that track different industries., and depending on which broker has your 401k, these funds are available to you.
The whole point of the 2008 crisis is that people created shady investment vehicles (mutual funds for example) on top of different securities, mainly debt tied, and this practice hid their risk; the debt holders were not paying their debts.
It is very possible that somebody's 401k went to shit if they put it all on a fund that tracked mortgage bonds for example.
Your friend probably didn't lie. There is a lot of misunderstandings around the 2008 financial crisis. There are also tons of shit advisors all around. When I started investing, I paid for a Vanguard advisor, and I was able to beat his performance after a year. Since then, I haven't let anybody make decisions on my investments other than myself.
I "lost" 40k this time only on my 401k, but it is all paper money until you cash out. I already made 20k of that back.