- You'd book the award tickets by calling UA, not on the LH website.
- If you credit to UA, you get 100% miles for all tickets-- on LH you might be getting only 50% miles for some of the Economy Class tickets you buy, so your 25% bonus wouldn't seem to help much unless you are buying tickets that give you 100% credit on M&M. Also, if you use UA, you could have Star-Alliance Gold status if you fly enough-- 50,000 miles in a year.
Isn't the 25% bonus of the base miles, so even if he gets a "cheapo" ticket he will still get 75% miles? (If you fly C, you only get 25% of the miles of the base miles (C gets 200%, so you earn 225% of base milage)
But of course it might make sense to credit them to UA if you want *A Gold... I did around 80k miles last year with M&M, quite close of reaching SEN, but still a bit away. If I would have done it by crediting to UA, I might be *A Gold by now, but having had earned less miles (Given I had a fair chunk of flights in C) compared to what I did. Also, keep in mind some of the extra milage programs that LH has won't work on UA (I got around 15k bonus miles from "Even More" and some more from "Aufgedreht")
Side question, I know there are a lot of fares on SQ which won't earn any miles at all on M&M (or KrisFlyer) - do they earn miles on UA, or any other *A program?