No, they are fully entitled to continue paying full price for the plan of their choice, but could not receive a subsidy if they are now medicaid eligible.
They could go on Medicaid, or they could find a way to earn a few extra dollars during the year so they can qualify for a subsidy and get a health plan. At those levels, it's only about $1,000/year they'd have to earn. I don't know about your client's situation, but it's not hard for most people to put in a few extra hours here and there through the year to get what they want.
If their income did happen to fall below the cutoff at tax time, there is no clawback of subsidy or APTC. No harm done. They try and renew the client into Medicaid next year, and you can repeat the process.
Let say that he's income was 100% FPL in 2014.
He signed up for 2014 exchange plan and received subsidy last year.
However, he won't be eligible for subsidy and have to pay full premium in 2015 if he's income fall between 100% - 137%. Or apply for medicaid and cancel the existing plan.
If you don't do anything, I know that Coverage under 2014 plan will automatically renewed and start on January 1, 2015.
Do you have to manually terminate your 2014 plan on healthcare.gov if you dont want to automatically renew your plan and sign up for medicaid?
Client I had in this situation received an 8-page renewal notice (from the exchange) that basically said "We cannot renew you in your current health plan. You need to select a new plan if you want coverage in 2015. You now qualify for Medicaid. Go get that."
I can't speak for other states (we're a state-run exchange, so it may be handled a bit differently), but I'd presume the same thing will happen. Can't renew, must enroll in medicaid manually if they'd like to continue coverage.