In Ontario, must a Decree of Divorce paper have a court seal to be legal?

The papers have to have been filed with the court in order to be legal.  Having, or not having, a legal copy does not have any bearing on the legality of the divorce.

She can have a legal copy of the court decree, but it will have a notation indicating that it is an exact copy or true copy or true replica or (as it would be in Quebec) "Copie Conforme".  Otherwise, it is not a legal copy even if it is an identical copy.  The papers, even if a legal copy, are not what makes the decree a legal decree.  It does not matter at all in law whether her copy has a seal or not.  The judgement can, and ought to be, verified with the court, if the purpose of the verification is to satisfy the legality of the judgement.

If a lawyer sends her a document and declares a legal consequence results from what that document indicates, then the lawyer would be in violation of the law to make a false declaration, so you can be all but totally certain that a letter from a real lawyer that says her divorce is official and she is free to marry again, is true.  No lawyer would risk his license, his livelihood and career, to make a false declaration on such a matter.

It's not a question of if the PAPER is "legal", whatever that might mean. Physical things by themselves are not legal or illegal. It is only what people do that is legal or illegal (or sometimes what they fail to do).

Some organizations need to see a proof of divorce, and will not accept a paper without an official raised seal and court signature. That doesn't make the papers without a seal "illegal", it just means that they are not an acceptable proof for that particular usage. The divorce is still a valid divorce, even without a raised seal paper.  The court records are the first and best source, but they are not the only source. Many people will accept a plain ordinary verbal declaration, "I'm divorced."

It's best to get an official raised seal paper, she never knows when she might need it. Getting one 40 years from now, when she applies for pension, could be a hassle.

P.S. there has been a case or two, where the judge granted the divorce, and then years later it was lost in the court records. Or somebody forged a divorce paper. That can cause much trouble.