It’s a touchy subject for most, either way it’s approached. And again, I don’t believe it would always go over well with just any stranger you just happened to see. - My original thought to the OP, was to try and suggest what the topic asked, i.e., ‘Tactful way to tell someone’. And my first thought was a T-shirt or something similar, because it might spark a conversation, which would actually then be that person inquiring. But this could be a hat, button, etc.
I also agree, that no one should ever feel something is being pushed on them in regards to anything, and we all know it won’t work anyhow, until that person decides they want to make a change. No matter what the subject matter. … But as I’ve mentioned before, if I’m completely honest, I do wish someone had just introduced it to me, many years ago! And yes, all people are indeed different, so you will never know what results may come?
But for myself, even if a stranger had sparked up a conversation and and no-so-tactfully brought up the subject, I would still have been appreciative. And if I wasn’t ready myself to take an interest in it at that time, I wouldn’t get upset, if I felt that person was just trying to be sincerely helpful.
On a side note, which I find coincidental to this thread. My wife called me yesterday morning to say that one of her cousins just happened to post something on FB, asking if anyone knew of a good diet to help him lose some weight fast. - So she sent him a private message, letting him know that I started doing Keto 3 months, and after seeing my results, both her and my son have now started as well. And that if he was interested, he could call me to discuss it. But she wanted to make sure I wouldn’t mind speaking with him. … Which I certainly don’t. Though I already have his contact information in my cell, since we’ve hunted together some over the years, I will still wait for him to call me, since it will be his initiative to do so.