How can I install a new internal nonbearing concrete block wall in basement to partition off for bathroom?
Load bearing or not, a concrete block wall needs a footing beyond what your questionably thick slab is.

That is why it might fall down, aside from not using tie-ins. AFAIK, you shouldn't have to tie-in at the ceiling, just to the side walls. I'd hope any professional would be talking about (a) pouring a footing (b) drilling holes everywhere for rebar and (c) making pockets for tie-ins. But it's a bathroom; not a vault. No one does this if they can just frame out the wall with lumber.

Ad hoc masonry must be tied-in to the existing structure:

 

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