I have a tile bit and I (think) my stud finder is good enough to locate a stud.
your stud finder is good enough to find the center or at least a trailing edge of a stud because you put new batteries in it? Come on bro, don't do something stupid here.
And even if you find centers of studs and can live with the angle of the safety bars to hit them (ADA guidelines will not match) you will still be hanging the bars and the person holding them on 2x4s toenailed with 16p if the framing contractor was cheap or 20p nails toe-nailed top and bottom? Just because there is an existing tiled wall there does not mean you can hang anything, including even a tiny person from it. Guessing given national trends, the need for a safety bar leans toward a more portly or substantial person?
As Bud Cline warns. This is rift with danger. Physical and legal.
I know you are trying to do your neighbor a favor but do so in a different way. Find him someone bonded and insured to do this sort of ADA work. He/she may qualify for direct financial assistance to get the bars installed---and the wall rebuilt or enhanced and tile in the process. You can supervise to make sure your neighbor does not get fast talked and ripped off.
This is not a DIY project. IMO. You most certainly cannot approach this with a tile bit and a questionable stud finder.