Failed cement grouting on patio wall

You need to chip the old crumbly mortar away and then reset in new concrete. You can buy bags of ready mixed concrete from diy shops. Much easier if you could find someone to do it for you. Have you tried asking for help on something like Next Door? There may be some kind soul who would do it for free.

Is there a similar produce I can use to fix the failed grouting on the wall which people sit on?

Who sits on it?
Could they do the job if you provide the materials?

I believe you'll need to chisel off old grout prior to re-setting.

They really need resetting in mortar, concrete adhesive relies on clean firm surfaces both sides, if you use it on old crumbly mortar it won’t hold.

Is there someone else who could do this for you? It does seem quite a difficult prospect. Surely family could help?

Concrete adhesive should do the trick. It’s available in most hardware stores. I used it to stick stone slabs that were coming loose on some steps and it worked a great. Good luck,