Normally I'd say no, don't tell her as it's a sensitive subject for many women, but then you added the 'spinach clause' & that could change things!
Only you would know if/when the right time might be, but I'd start with complementing her similarly to what you said in your post...'She is attractive with a great figure and hair... and looks even better now than she did 10 ears ago' (assuming years not that she's had multiple ear transplants!)...
And then add something about the spinach clause & you'd hope she would tell you if the heel was on the other foot but... 'she looked so naturally beautiful & you didn't even know/realise she'd been getting Botox (she just looked so great xx weeks/months ago), buuuuutttttt, now it is a bit obvious that she's getting Botox treatments & if that's the look she's going for then great, she's succeeding, but if she was going for 'gracefully putting us all to shame' (ha ha, it's a wee joke, I'm sure you all look gorgeous!) she's gone one (or however many) Botox treatments too far...(very gently etc etc).
Some other posters have had great suggestions too & whilst we never fully know one's motivations for doing things like this, you deffo don't want to make her feel worse if she's struggling with something (IE, being single, depression, comments from someone - especially a partner, a younger work environment, etc).
You're clearly a loving friend & don't want to see her (in your opinion) either become overly dependant on Botox or start to look so fake it defeats the purpose/goal of carefully doing Botox in the first place...
Give some thought to how you'd honestly feel if someone told you something in the same vein... clothing style, make up, weight, etc & how you might 'hear' such a friendly, well intentioned comment. Again, receive is different & this includes possible reactions...
It's a delicate situation & one you don't want to cause offense or deep hurt, so certainly give it plenty of thought before proceeding along a course of action or inaction... Best of luck & I think you've got a small but if spinach in your ear...😉x