So they end up paying 50k to buy some common shares (not preferred). An investor who paid 50k to invest in the company most likely got preferred shares, so the young guy who paid 50k who probably can barely afford that is now taking way more risk for a much smaller percentage of the company. If the company goes under the preferred shareholders have a chance to get their money back during a firesale of assets or IP or whatever, but the common shares are screwed.
So I tell virtually everybody unless there is a well established secondary market to sell your shares of your particular company, then don't take the options.
I honestly think the 90 day exercise is totally ludicrous in the startup world. I think that should be a major negotiation point with anybody who is joining a startup. They should just insist on it no matter what.