I'm sorry you and your son are going through this difficulty. But if your daughter-in-law was certain of her love for your son, she certainly wouldn't want more time away from him. Regardless of whether she's found someone else, she doesn't want her husband. Encourage your son to mourn the loss and then keep himself busy with starting a new life of his own. Even if she has a change of heart, he may decide that he likes his new life without her.
All you can do is be there for him by advising that "This too shall pass", point out his good qualities, and encourage him to go on with his life. If she's as determined to end this marriage as she seems, she probably will refuse counseling. Your son's circumstances surrounding his job are no fault of his own.
I have two younger sisters who are married to Navy men. They spend months apart from their husbands. Your daughter-in-law could have it much worse than she already does.