If your medicare coverage starts on 2/1/25, then your maximum eligible HSA contribution for 2025 is 1/12th of $8550 plus 1/12th of the $1000 catch-up, for a total of $795 ($712 plus $83). This contribution can be made any time in 2025, even after medicare enrollment starts, as long as you don't go over your eligible limit for the year.
If your wife is covered by your family HDHP for all of 2025, and has no other disqualifying coverage, then her HSA contribution limit is $8550 plus $1000 catch-up if she is age 55 or older. The $8550 limit is shared between you, so if you contribute your maximum of $712, then her maximum from the shared limit is reduced to $7838. The $1000 catch-up is individual, so she can contribute $1000 for catch-up even if you also contribute your $83 catch-up. So her ultimate limit will be between $7838 and $9850, depending on her age and what you contribute.
If you drop your HDHP coverage with your employer and your wife is only covered by an individual HDHP, her limits are about half what I wrote above. But I think this is unusual, most businesses require you to keep family coverage in order to cover a spouse.
Also, note that her limit depends on the number of months where she is covered by a family HDHP on the first day of the month. If you terminate on or after December 2, then she has 12 months of eligibility, but if you terminate on November 30, she will have 11 months of eligibility and her limit would be adjusted accordingly.
Lastly, if she is over age 55 now, she can open an HSA in 2024 and contribute her $1000 catch-up contribution for 2024, even if you have maxed out your shared family contributions.
