If it is a mere opinion, it would not constitute libel. For something to be libelous, it must be an assertion of fact - it must also be false and tend to injure the reputation of the party within the community.

The company would here argue that her tweet asserts that company policy deems it acceptable to expose tenants to serious health risks. It would say this is false and highly injurious to its reputation in the community.

Had this firestorm not arisen, the company might easily win through intimidation in such a lawsuit by coercing a settlement. A court could let the case get to a full trial, meaning that it would (if nothing else) take many thousands of dollars to defend it.

For this reason, it is not wise to use Twitter as a forum to vent in this way.

That said as a general matter, in this case, the company's patently idiotic response did far more to hurt its own reputation than the original statement possibly could have done. Unless it has lost its mind, it will drop the case pronto.