I'm going through this with my parents too (dad still alive but not cognitively capable of investing, mom has not been involved in any financial decision in their half a century of marriage). My advice is this:
1) as you would with yourself, consider both her ability and her need to take risk when setting an AA so you can prepare for the worst case scenario. Also, her goals matter too. Is she ok with spending every last penny, or does she want to leave an inheritance? You didn't say how much her stash is relative to her needs, but maybe a 6% WR is ok for her life expectancy if she doesn't care about leaving an inheritance behind. Once you know what her goal is, you can do some calculations using a site like Firecalc to figure out her odds of success at that WR.
2) involve her and get her input as much as possible, including educating her as much as she will or can learn about investing (may not be practical but you still should try).
3) consider hiring a fee-only planner, not because you aren't capable of doing it, but, as you alluded, the stakes for failure are so much higher, and there can be a lot of family emotion surrounding mom and dad's money. Offer to go with her to meet with the planner, be involved, etc., but consider having the decisions made by someone else so that there isn't ever any question of you having a bias or vested interest later, let alone being "responsible" for losing all of mom's money if something goes wrong. This is especially important if you have siblings or other relatives whose inheritances might be affected by your decisions - even if money isn't an issue between you and your siblings now, it can easily become one in this context.