As far as I know, there are no actual 1/6 scale Transformers, beyond maybe some customs I've seen made in Asia out of junk metal. If a real Mac truck is 69' across in truck mode, then a 1/6 scale Optimus would be 11 1/2 feet long!
But as far as 12" figures, there is a Hot Toys Optimus inspired by the G1 cartoon that has reasonably good reviews, though he can't transform. 3A has done some from the Bay movies, though I don't know much about them--they also can't transform. Also, the original Masterpiece Optimus and Megatron figures (which could transform) were about that size.
Instead of looking for something of a certain size, it may be better to ask your friend what era of Transformers he is interested in, and then folks here could tell you the best available options in terms of modern toys. For G1, I can say without hesitation that, if you want great looking figures in high quality that do transform, the Takara Masterpiece line is the way to go, though he should stick to figures created after the reboot of the line with MP-10 (smaller Optimus).