A Power of Attorney (POA) is a document in which you appoint someone else to act on your behalf on matters that you specify. A POA can be made to start immediately, or upon mental incapacity and the authority granted in a POA ceases at the death of the person granting the power.
For the average Marine who pays a car payment, cell phone bill, car insurance, etc. each month, s/he is given guidance and an opportunity to get those things lined up with his/her payroll and automatic payments while deployed. Even taxes don't have to be filed during deployment. Cell phone companies generally want a copy of the orders to suspend service, but all of these arrangements can be done prior to deployment.
With that said, a Power of Attorney is not standard for parents or spouses. A POA is rarely a required document and Marines have the right to say, "No, I don't want to give anyone a POA" in order to protect their own interests. A Marine should get his affairs in order prior to deployment and therefore won't need a POA. The exceptions would be for military personnel who have estates, own a business, or have more complex issues with finances.
Some parents may be specifically asked to pay bills for their deployed Marine, and may have a hard time getting account numbers and cooperation from banks and institutions. Clearly it's good news that these institutions are protecting your Marine's financial interests on his/her behalf. When it is necessary, the Marine Corps has facilities in place for your Marine to complete a Power of Attorney (POA) prior to deployment.
If your Marine had completed a "Power of Attorney" prior to deployment, you would have been mailed a copy. In addition, unless your Marine told you s/he put a POA in place and you have received a copy of the POA, then you do not have Power of Attorney and you would therefore not have authority to act on your Marine's behalf. If something requiring a POA comes up during your Marine's deployment, they can generate one overseas and have it sent via electronic means to the appropriate parties stateside.
We encourage parents to advise their Marine to take care of personal and financial affairs before deployment, in which case a POA should not be needed. The Marine Corps philosophy may differ slightly, especially for Marines who are married and/or have dependent children.