It's incredibly tough for any of us to consider that our 18 year old sons and daughters are faced with the task of writing or having a will written prior to deployment, but it's a fact of military life and there is no way around it.
Your Marine will be given an opportunity to write a will prior to deployment. Depending on the verbiage of the will it may expire at the end of the deployment, or it may continue after the deployment has concluded.
Generally, the basics of survivorship are addressed in the standard wills written through Marine Corps channels. If your Marine has other issues that need to be addressed, please discuss this with your Marine prior to deployment.
Depending on the Marine, s/he may or may not choose to have a discussion about writing a will with you. Remember, your Marine is an adult making adult decisions and that responsibility should be respected and allowed without interference.