Yes agree this is a situation where you need to be honest; and your friend will thank you for it in due season too.
Obviously you will need to choose your words, but you could be diplomatic and say well maybe she needs to wait awhile/see one of two other horses first etc etc; and/or say how nice it would be NOT to have all the work and hassle of a youngster but have something a bit more push-button etc etc.
The main thing, for now, if you can, is to dissuade her from making a silly decision which she might later regret very much.
She's obviously in a state of grieving for her old horse, who wouldn't be, and being in a state of grief is never a good place to be for making a decision about anything, let alone a life-changing thing like a horse.
Perhaps (this is just a thought) she needs a diversion? Maybe you could invite her along to a horsey event near you (dunno where you are or what's on in your area, if anything) but it might be for e.g. that you could maybe suggest you both have a go at an equestrian discipline which is new to both, say western riding/endurance, or go for a lesson together??? This just might lighten things up for her a bit??? Plus give her a bit of an impetus maybe. Or perhaps go as spectators to something?? Just a thought.
OR you could suggest to her that as getting another horse is obviously important to her, the choice needs to be the right one; and if there's a good dealer or agent in your area maybe go along to their yard with her - as a GOOD dealer/agent will help her to make the right choice and will do everything to that end.
But whatever, I do think you need to be as honest as your friendship permits; she's had an awful upset in her life and may not be as prepared to listen as you might hope, as least initially, but I do think you need to be as realistic with her as is possible at the moment because the one thing that would be worse for her than no horse, is the wrong horse.