How is he cooking them? Has he tried marinating them in some Worchestire sauce and some seasoning? It will help keep it from drying out. BTW some people had some good suggestions when it was brought up before with seasoning it and cook times. You might want to check out the old post.
If he's making them from scratch with ground turkey, he should try adding chopped onion and some worchestire sauce in with the meat - both add moisture that you lose when you use low-fat meats. If he's feeling adventurous, he could try adding some chopped herbs or green pepper as well.
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I made some really good ones the other night where I mixed the turkey with some shredded zuchinni and chopped jalepeno. Don't have the exact recipe here, but I'm pretty sure that was everythin in it- maybe some salt and pepper? The zuchinni made them really moist.
i always add olive oil, plenty of salt a pepper, and some pureed garlic (if you have a microplane grater, which everyone should have, it makes this part easy). i also sometimes add soy sauce. my special trick with burgers is to put a hunk of cheese in the MIDDLE (rather than on top at the end), so you don't have to worry about the middle being undercooked--and therefore don't overcook it and dry it out. plus when you bite into it there's a warm, melty, cheesy surprise in the center.
this recipe is really good -- the cheese helps (like mizzle said) and the pepper relish adds a lot of moisture too.
from epicurious.com:
GOAT CHEESE-STUFFED TURKEY BURGERS WITH ROASTED RED PEPPER RELISH
The goat cheese and relish keep these burgers especially moist. For a nice variation, omit the goat cheese, and top the turkey burgers with sliced Havarti or Monterey Jack cheese during the last few minutes of cooking. Set out bowls of pickles and olives, and pour chilled Chardonnay throughout the meal.
For Burgers: Combine turkey, breadcrumbs, lemon juice, lemon peel, thyme, salt and pepper in large bowl. Mix well. Divide turkey mixture into 6 equal portions. Form 1 portion into two 4-inch diameter patties. Place 1 tablespoon goat cheese atop 1 turkey patty; place second patty atop cheese. Seal patties at edges to enclose cheese. Repeat with remaining 5 portions. (Can be prepared 4 hours ahead. Cover and refrigerate.)
Prepare barbecue (medium-high heat). Grill burgers until cooked through, about 5 minutes per side. Grill hamburger buns, cut side down, until lightly toasted, about 1 minute. Place turkey burgers on bottom half of buns. Top burgers with Roasted Red Pepper Relish, then bun tops and serve.
For Relish: Heat oil in heavy large skillet over medium-high heat. Add red peppers and sauté 2 minutes. Add onions and garlic. Cook until onions are tender, stirring frequently, about 5 minutes.
Mix vinegar and sugar in small bowl until sugar dissolves. Stir vinegar mixture into red peppers. Mix in mustard and cayenne pepper. Season with salt. Continue cooking relish until all liquid has evaporated, stirring frequently, about 6 minutes. Cool to room temperature. (Can be prepared 1 day ahead. Cover and refrigerate. Bring to room temperature before using.) Makes about 2 cups.