There are some hypersensitive retards in this thread! Of course I'm aware that Walt used to meet Lydia at the same table, in the same café, at the same time every week (he didn't know she did it with Todd, rather just assumed) and that's how he'd get the chance to poison her with the ricin. A fecking braindead chimp can deduct that from watching the show. feck, Walt even spells it out for her in the phone call at the end of '
Felina'.
The part that is a little ridiculous is how Walt got the ricin into a seemingly normal sachet of coffee sweetener and had it re-sealed perfectly, with no visible perforations. It was a paper sachet after all.
Also, how did he know she, or the waiter, would choose that exact sachet? That's probably even more difficult to execute.
Why can't people just accept that it is an example of
deus ex machina/plot hole without reacting like the person raising the issue has shagged their grandmother?