Thanks for your question
The responsibility lies with them to get VAT registered, and consider the fact that this may need to be backdated (based on what you advise)
But it would do no harm to point this factor out to them, that they are in fact in breach of the threshold on a 12 month rolling period.
Just a last factor, if the income that they take from you, is such a level, this does seem to suggest that you might be their only client, (or a main one at least with continuous income) and I have a little concern that maybe they should in fact be an employee and not a consultant. But I am sure you have considered this and reviewed the employment status legislation.