Hello,
1) If the lady's family hid material facts at the time of marriage and lied about her age brings in an element of deception in getting the alliance completed. However in the preset case of your cousin it is not ground enough for filing for divorce.
2) The best way if the couple find not compatible with each other is to file for a divorce by Mutual Consent.If the wife is agreeable they can move a joint petition before the court for divorce .
3) Now as they have a one year old child in the marriage, your cousin will be liable to provide for the maintenance of the child and the mother as well especially if she is not capable of maintaining herself.
4) If the wife consents for a mutually consented divorce, draw up a consent term and include all issues pertaining to maintenance, alimony etc to form part of the divorce decree.