ATP wants a single engine add on

And for some reason he's picked me for the honor of training him.

But I'm a little confused on one thing. He claims to have a
commercial-multi, limited to centerline thrust, and an ATP multi,
unrestricted, with 737 and 757/767 type ratings. I wasn't aware that
you could have both those certificates...is that true?

And if it is, onto which certificate does the single engine airplane add
on go?

And one last question. I know I'd never, ever, train a primary student,
as long as it's been since I instructed, but this shouldn't be a huge
deal, right?