If the employer has less than 20 employees, they will need to enroll in both A & B when they turn 65. If they don't, they will incur a 10% penalty on their part B payment. In addition, Medicare would be considered primary insurer at that point, and the employer plan could refuse to cover services that should have been covered by Medicare.
And just so the OP doesn't get confused, it's 10% per year that they didn't have part B. Not just 10%
Also, I feel really bad for this posters clients. They are most likely on a high priced, high deductible small group plan and he can't explain to them how beneficial it will be for them to pick up Medicare and a supp.