People in general are sheep who are unwilling to go out on a limb unless led. As the saying goes, "you can lead a horse to water, but you can't make him drink," well, with most people you end up finding out the horse are thirsty, it tells you every day, but while you can tell them there's water over the hill, you have to bring them to where they can physically see it before they'll move an inch on their own.
Props to you for being a great coworker. I second the notion that you should get them to understand the ebbs and flows of the market so they don't panic and sell (and, worse, trigger early withdrawal penalties) if they get cold feet during a downturn. If their contribution rates are high enough, they'll never even realize it's down, to be honest. But they should transition to less risk exposure over time, so the best bet would be a life cycle fund, if one is offered. If not, it might be worth it to tell them to throw a small percentage (5-10% depending on age) into a stable value fund, and just explain that this is their safe haven which will never really decrease (since these typically invest in treasuries and short term obligations), so they will never get "wiped out" unless they sell out. Then explain that the stock fund should do very well for them, but if there is a downturn, will go down temporarily, and eventually go back up again. While that's not the optimal mix, looking at where we are in the business cycle and the psychological effect on people who are less experienced with investing, it might be a small price to pay for the long term good of keeping them in the market whenever things hit the fan again.
I too try to advise people at work (or personally) to save & invest, and stay out of credit card debt. etc, even offering to look at numbers for things if they'd like (such as "should i buy this house" or "is this a good deal?") - but in general, people want someone to sit with them and walk them through the process. In my current role, I can't always offer to do that with employees I supervise (which just feels like it walks a border of what is proper or improper, unless they come to me specifically, of their own volition), but that's what most people seem to want. There's a coworker I've got to talk to about her account specifically (not someone I supervise) who has expressed interest in it in the past, and I figure next time I catch up with her it would be good to check in and see if she was able to enroll, or if she needs me to actually walk her through it.