I have a friend....yea a friend....that had the same situation. They wrote the letter and the signer had final edit before it was sent confidentially to the school. The final letter could have contained anything.
I don't care what u read here, this happens ALL THE TIME. The people who can write a letter are frequently too busy to write a good one. I know one doc who makes his candidates write a letter, just to see if they want one bad enough to go to the trouble. Plus, it gives him valuable information about them.
If you want to feel ethically ok, do what my "friend" did.
Edit: quick tip, there are plenty of old letter writing instructional texts at the public library that can help with tone, language and flow. Trust me, they are more valuable than you might imagine.