They would each have to submit form CF83 to HMRC and soon, as the already extended deadline for years 2006/07 to 2018/19 to pay at all and for those years plus years up to 2023/24 to pay at the current rates expires on 5 April 2025. But as long as HMRC has the CF83 before that, the deadline is essentially extended as HMRC has said people will not be disadvantaged by their delays.
There is absolutely no downside to sending form CF83 as it doesn't commit you to doing anything - this is all voluntary.
Depending on their age, they my have the 3 freebie years as long as they turned 16, 17 and 18 before 6 April 2010. To pay NI voluntarily you need either 3 consecutive years of UK residence or 3 years on your NI record. On the surface they may qualify on that basis but HMRC will confirm the position.
Then, if you worked in the UK until immediately before you left, you open the door to pay at the cheaper Class 2 rate. If the Class 2 door is opened, then for any week they work abroad (Germany or elsewhere) they can pay Class 2. Otherwise it would always be Class 3 (what I have to pay as a UK resident).
I would:
- get the to set-up a Government Gateway ID using their NI number
- get a copy of their NI record
Discuss on here what that all means and see what HMRC come back with.