Does he want a huge wreath, or just some type of floral arrangement on the altar?
I have to admit, from what you described, my first thought was of a funeral (with those memorial wreaths on metal flimsy easels that they group at the front of the church).
Now, I hope this doesn't offend, but I am in the Army and have lost many friends. I can sympathize with the Fiance' about the importance of his fellow Marines. I do, however, believe in a time and a place for everything.
You would not put a 16x20 or larger framed print of a departed mother/father/brother/sister/grandmother/grandfather on an easel at the altar of your wedding. So, why such a large gesture as a wreath?
Most brides on here who have departed loved ones honor them with memorial vases, flowers on seats, etc. And you cannot say that those loved ones are less important that fallen Marines to the bride and groom(in fact, that would be pretty absurd).
I think there are better ways to honor what is in his heart without making it the centerpiece of the ceremony. After 11 years in the Army (and being married to another Soldier), I can say that the Army factors large in our marriage, but in no way does it define our marriage. Most people spend 4 years or so in the military. Some go to 20. Very few go beyond that. After that, you still have your family for many, many years.
Do you think he would be agreeable to having a memorial candle on the altar? Then, when he enters with the minister, he could go to the altar and light it. You could have a note in the program that explains the candle as a symbol of their presence with us. I think that would be more poignant without people forgetting they're at a wedding CELEBRATION.
To all those who say she should get used to it or let him have it, I say this: there will be too many times when the military comes first not because it should, but because it DEMANDS to come first. He will miss anniversaries, birthdays and probably the birth of their children. I don't think it's too much for her to ask that this day be about the two of them and NOT the Marines.