She asked if he can enter Canada and If he can come here on his own; Yes he might. He cannot use their marriage to enter Canada without her sponsoring him. But he can enter Canada with other immigration streams. There have been people who have been given an exclusion order, deported etc, and once their ban period is over they apply again (might need an Authorization to return to Canada). He left after his visitor extension was refused in 2015 he was not removed by force (he might not even need to apply for an ARC). He was denied entry once, depending on why he may have been denied entry to Canada, he may be able to re-apply for entry (especially if it was a voluntary withdrawal).
I am thinking that he did not get the 5 year ban because she was trying to sponsor him last year (so I assume he is admissible and met the conditions to be sponsored?). Well she did not give enough information about this, but the main point is while he cannot enter Canada using their marriage without her sponsoring him(if this is what she is worried about), he can still enter Canada using other immigration programs (will definitely be difficult for him)
You should process a divorce in Canada. You do not need him to consent