Yes, you can file an amended return for your son-in-law, changing his filing status to Married Filing Jointly and add your daughter's information to the return.
You don't have to wait until after April 15 to file an amendment, but you need to wait at least 21 days after his original return is accepted.
If it's not practical to get the amendment done before April 15, you should file an extension for your daughter.
If your daughter's student loans are in collections, Form 8379, Injured Spouse Allocation can be filed with the return to protect your son-in-law from having his refund applied to her debts.
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