What is a good subsitute for Mirliton (aka Chayote)?
Mirltons are a member of the squash family, but have a traslucent, soft-crisp texture which isn't really exactly like any other squash.  In recipes where they are an incidental ingredient, you can generally substitute zucchini (especially if you peel and seed it) without too much trouble, but in recipes where they are a main flavor they are hard to substitute.

And, as Mien points out, it never really works to substitute the main ingredient in a recipe.  I suggest that your daughter wait for midsummer and check out Mexican or Indian groceries until she finds some.