Disney's price for a 7-day MYW Base ticket (expiry) is $223.65/$184.25 (adult/child). A 10-day MYW Base (no-expiry) is $395.12/$353.58 . Your friend would pay the difference between the two, which is $171.47/$169.33.
FWIW, the no-expiry option for a 10-day pass is $168.08, regardless of the number of days which have already been used or whether the ticket is an adult or child pass. By adding 3 days and the no-expiry option to their tickets, your friends will be paying $56-$57 per day for those extra 3 days. If they don't plan to spend more than 3 days in WDW on their next visit, this isn't such a bad idea. If they will need to buy another ticket to cover extra days, then doing the addon days and no-expiry will end up costing them more money in the long run.
For example:
To add 3 days and no-expiry to their 7-day MYW base ticket, an adult would pay an additional $171+. If they'll be going to the parks for 5 days on their next visit, they would also need to buy a 2-day pass for $140.58. The total cost for their 5 days in the parks would end up being $311.58. But the cost of a 5-day MYW Base ticket (adult/expiry) is only $219.39.
