First of all, it should be understood that divorce given during pregnancy is just as valid and effective as it is given in other times. However, the Iddah period for woman who is pregnant and divorced is her delivery of baby.
Now, if your cousin gave divorce to his wife only once or twice and he had not given divorce to her before at anytime during their marriage then this divorce is considered revokable (Raj'i) and he will be allowed to take her back by revoking the divorce as long as she has not delivered the baby.
If he gave her divorce three times or more then the divorce will become final and he will not be allowed to take her back as wife unless she marries another person after delivering the child and then get divorced by the seond husband. Only then, she will be allowed to remarry this person.
The above ansnwer is given based on what has been presented in the question.
And Allah knows best.
Mufti Ikram ul Haq
Fatwa Center of America