Will my client incur Medicare Part B late enrollment penalties if they have premium-free Medicare Part A and enroll in Marketplace coverage because it is cheaper?

It depends. 

If the client became eligible for Medicare Parts A and B before age 65, any late enrollment penalties they may have incurred (as a result of deferring Part B coverage) will automatically reset to $0 when they turn 65 and become eligible to enroll in Medicare again during their Initial Enrollment Period.

If the client became eligible for Medicare Parts A and B at age 65, they will likely face a Medicare Part B penalty if they choose Marketplace coverage instead of enrolling in Medicare when they first become eligible during their Initial Enrollment Period. They will be ineligible for Marketplace subsidies, such as Advance Premium Tax Credits (APTCs) and Cost Sharing Reductions (CSRs), which will increase their monthly premium amounts.