My mom is retiring and qualifies for medicare. She is in excellent health. Does she need supplemental insurance?

If she's enrolling into Medicare Part B, for the first time, then she has 6 months from the 1st day of the month of her enrollment to qualify for ANY Medicare Supplement Insurance Policy.  After that period of time, she will need to be medically qualified to get a plan - she may be healthy now but that can quickly change and it is possible that she would then not be able to get a Medicare Supplement.  There may be some slight variations to this rule, depending on the state of residence.

Don't forget that regular Medicare (Parts A + Part B) does not have a limit on out of pocket expenses that can be incurred.  A supplement policy can be a vital tool in protecting one's assets from medical expenses.

Take a look at the Medicare Supplement/Medigap Guidebook:

https://www.insuringmyself.com/files/2015-Medigap-Guide-to-Health-Insurance-for-People-with-Medicare.pdf (published by the Centers for Medicare & Medicaid Services)