CFP here, good question. You are on the right track though I have a couple of questions to ask the executor:
1. Were these direct purchase shares, or otherwise in a taxable account? If so, did these go through probate? By now the shares should already be owned in an individual account by your mom. If not, you will need to contact the custodian and provide a death certificate, probate documents, and probably their own form.
2. She received a step-up in basis at his DOD, presumably to a value higher than when they were purchased. What has happened since his death is irrelevant to the basis, though worth comparing to the original cost basis to get a true picture of the gain/loss. Inherited shares are always taxed as long-term capital gains, though in this case if she sells she would realize a loss (which may be valuable in terms of tax-loss harvesting and carrying forward the loss to offset up to $3,000 in gains in future years).
You may have heard of the 'alternate valuation date', which is a way to re-value assets six months after death in the case of declining values. However, this applies to the whole estate (i.e. home and anything else he passed on) and MUST be elected within one year of filing the form 706, so it appears are well past this deadline.
This can get complicated quickly, so let me know if it doesn't make sense. If your mom does nothing and holds them until her death, then you (and your siblings, if any) would receive a step-up or step-down in basis as well, which could be good. Depends whether she needs the money and the long-term prospects for the stock.