This is exactly the problem. An Air with Lion, an SSD and 2GB of RAM was usable due to fast swap. A Mini using a laptop hard drive, not so much. Upgrading to 4 or 8 GB will be a massive improvement.

Also, Word is not the most memory friendly or speedy due to it's continued usage of the Carbon API, the older OS 9 based API Apple included to allow devs easy porting to OS X. Unless your mom needs 100% exact Word compatibility, Pages may be worth a look to gain some speed and memory efficiency.