A friend had a low brick garden wall built with slabs on top for seating.
A couple of slabs came loose after very little use and it is evident that
none of them are fixed properly - a sharp tug will make them come off.

Builder is blaming friend, for having the temerity to sit on the seating
area. (Yes, he was told it was for seating.)

It doesn't help that the slabs are wider than the wall; the overhang
allows leverage forces to pull the slab up.

I'm happy to have a bash at fixing, my question is, what mix of cement/
sand/whatever should I use to obtain maximum "tensile strength"? Any prep
on the underside of the slabs? They are smooth-ish - think slate texture -
but fairly thick.