You should really have an ATP evaluate them to determine what they need. Hemiparesis and a bad fitting chair is no joke. Unfortunately this is likely to be expensive (as noted in this entire post). You can try getting a lightweight chair like a TiLite or Quickie online, but the fitting and sizing is the most important part and in my experience a trained professional is the best route your first time around. After that, as you become more familiar with the needs of your parent you can make adjustments in the future.