Downgearing with a road triple
The triple is irrelevant. So long as the derailleur has enough capacity(which is determined by the size of the chainrings and cogs in the back; this is basically controlling how much chain length derailleur can keep tensioned) and largest cog ratings (for clearance ratings), you can put a bigger cassette. 

There may be other ratings on the data sheet -- minimum cog rating, and maximum front chainring difference. These aren't really important. Exceeding the maximum front chainring difference may increase the chance of dropping the chain since the derailleur will need to take up some amount from a front shift. 

The capacity is generally conservative -- if you don't use combinations like small-small, you can exceed whats written on the derailleur. Obviously, the derailleur needs to be matched for compatibility to the shifter. 

That being said, you may also want to switch to smaller chainrings depending on the size of your chainrings. 

Another item might be switching the type of bike your mom is riding -- if she has a regular road bike, a more relaxed geometry like a dutch bike may be more comfortable and ridable. Spending some time working out bike fit may also pay off, even with the existing bike.