Iridium phone service is around $1/min, and drops when you add minute-bundles. The device will be around $1250, plus some accessories. Subscription is another $50/month.

Do they really need it THAT badly, or do you just want the option to stay in touch?

Many of the larger cruise lines offer GSM roaming on the ship, but you are right - at $2.50/min, that is insane.

I used Iridium for several years, but then GSM became even more available so I started leaving it at home.

It will be tough to get a good signal unless you are outside with a clear view of the sky, but Iridium is by far their best option. AFAIK, Globalstar is STILL mostly out of operation for voice calls. Inmarsat is much more expensive.