Elderly mother cant get poo out. What do we do?

If it has been more than three days and no poo, get to the ER today.

I was going to say a glycerine suppository or fleet enema, but I guess it really depends on how often this has happened and for how long she hasn't been able to go, also on what you have tried up until now.
Step one: extra fiber like prunes and whole grains and plenty of fluids
Step two: add in an OTC laxative product like miralax and/or colace and plenty of fluids
Step three: move on to harsher laxatives like bisacodyl or senna and plenty of fluids
Step four: glycerine suppositories and lots of fluids
Step five: bisacodyl suppositories or enemas
Did you get the part about lots of fluids? LOL

Prune juice

Stool softeners.