Lots of before photos, maybe draft something up stating that you intend to not harm the sidewalk anymore then it already is but that some further deterioration may occur that you will not be responsible for, and have customer sign it. Pretreat, surface cleaner, wand where necessary, post treat, after photos, collect check. Certainly exercise caution/care on visibly damaged areas, but bleach and real light pressure shouldn’t destroy it.
BUT…there are easier ways to make money. If you can’t say no to this job then go for it. But don’t be afraid to walk away.