So. What do you think? How could she (try to) kill him? (Will she do it? That I don't even know yet --and I have a very pushy lady here who wants to find out! )
What I want from you is some brainstroming. Ideas --whatever comes into your head. Remember it must be like and accident!
People took what we considered superstitions seriously. Maybe a lock of his hair would allow her to cast a spell. Whether magic exists in your universe (fantasy-vs-history) or not, pre-industrials will beleive in it. Historically speaking, IIRC, in the 25-50 years after Newton, witch trials ended in England: reason replaced superstition. (Of course, the backwards colonies like Salem still had witch trials...)
(An interesting twist to any fantasy would be that the magic isn't magic, but the work of skilled charlatans and alchemists -- history, not fantasy.)
If she's sufficiently upset about loosing her husband and doesn't care if she lives or dies, a nice open-hand strike to the larynx will kill anyone -- quite painfully.
Maybe she could first spend some time in the bad part of town "working" and figure giving the prince a "social" disease would kill him slowly, but destroy his public image as a bonus.
The two adjourn to a dark corner of the banquet hall, his chambers, etc. He takes a bite of food, swallows... WHACK! She hits him in the throat with a karate-blow. She then starts screaming, "Somebody! Quick! He need the heimlich manuver! He's choking!"
She'd better be damn sure it kills him, though, because if he lives, he'll tell everyone exactly what REALLY happened.
This assumes there are no skilled coroners in your setting, of course.
If we are assuming that she is not particularly stronger than him, there are two possibilities for an "accident" that come readily to mind. Both are helped by getting him at least a little drunk first (which is a problem, since if he thinks that he's got a shot at seducing her, wouldn't he rather get her drunk?), but don't actually require it.
First, persuade him to have her sit on his lap while he sits in the window sill facing into the room(this plan requires some height, of course, to be effective, but the nicest rooms are always the highest above ground, for some reason). Then simply have her push herself violently off his lap, and guess where he goes? Got it in one.
Okay, number two, much more exotic. Have them eating and making out at the same time. She takes a big bite of meat and spits it down his throat (he needs to be distracted by something, say an oversized goblet of wine, so that he doesn't realize that she's about to kiss him with a big hunk of meat in her mouth). This is a tricky maneuver, but has the advantage of really looking like an accident, both before and after. A variant would be to use an herb that is not poisonous as such, but causes swelling of exposed mucus membranes, then shove some meat down his throat after he chokes. But this is quite likely to be detected if anyone looks for it.