Likely plan: $29.99/month (though some might try the $15 lite-user profile)
Likely BW usage: At least 10-15GB/month, but if they expand into video or streaming audio could max that out with one movie download or a couple days of streaming audio
Online time: 1 computer at 7mb/s, capped at 10gb 3 ACTIVE hours of online time per month or 6 hours of total usage per month (assuming 1/2 the time is spent as wait time)
Cost of saver plan: $15 + $2 * 14GB of usage = $15+$28 = $43/month
Cost of lite plan: $30 + $1 * 5gb of usage = $35/month
Possible max cost of either plan: $150
Biggest dangers: Discovering new bandwidth intensive services, without realizing impact, or having their computer system compromised or their wireless access used without their knowledge could easily max out their bandwidth, which they wouldn't find out about until they got their monthly bill.