While MA is an at will state, an employer cannot fire an employee for an illegal reason. An at will employee can be fired for any reason that is not illegal.
Generally, absences even for illnesses are not covered unless they are protected by the Family and Medical Leave Act, the American with Disabilities Act, worker's comp, or state law. A serious illness can include an illness that lasts longer than 3 days and requires ongoing medical care.
Otherwise an employee may fire someone without cause. They will normally provide "warnings" and document incidents to help eliminate the chance of a lawsuit, particularly if a person is in a protected category.
If the employer made comments stating a person is doing a great job, that they can work there as long as they want - then it can be argued that there was an implied contract, and that can be enforced.
These are the protected classes: race, color, religious creed, national origin, sex, sexual orientation, handicap (if qualified for the job), age (age 40 or greater), genetic information, or ancestry.
So if any of those apply it is easier to make a claim for wrongful termination if there is insufficient evidence to support a discharge.