The SMG tranny is service free. BMW used...
a lifetime fluid. However...
If he has a ton of miles on the car, or is experiencing shifting problems, then I suggest that you contact Bavarian Auto and ask them which fluid they recommend. Then you can contact this forum about how to do it.
There is no tranny dipstick so there is no way to check the level other than via the fill plug.
If you are in the Modesto, CA area, I'd be happy to let you use my lift.