I have carried RX meds bought in Mexico for which I had no prescription several times. The sometimes ask if you have insurance or if your doctor wrote a prescription for the drug but most of the time do not ask. It is illegal but the government has a unwritten policy to ignore it. You should definitely disclose if asked that you bringing anything with you. Volunteering that they are carrying it for someone else will probably get it taken away. I highly doubt that the would care at all if you brought a supplement like mutaflor which is unapproved in the US unless it was well known to be dangerous or something you get high on. I am actually not sure if bringing a unapproved supplement is even illegal if you are just bringing for your own use.

If your friend is Canadian, I might tell him not to do it unless he is prepared to lie and just put an open bottle in his shaving kit. Maybe he could get away with pretending ignorance about mutaflors status here but the first thing they will ask is why your friend doesn't just buy it in the states. I would just have him mail it with your name on the return address. Both countries are pretty humorless about the others citizens disobeying the law. The crossing at Niagra falls has some of the biggest jerks in customs for both countries. I used to cut through Canada going to New England from Michigan. Hating going through there.

A small amount might not be an issue. Saying they are for someone else will raise a flag.

Coming for a day or weekend with a month's supply might also be a problem

/help.cbp.gov/app/answers/detail/a_id/1217/kw/food%20supplements/session/L3RpbWUvMTQxNDY5OTcyNy9zaWQvd2tSUXJiNm0%3D/suggested/1

I wouldn't worry about it or disclose it. They can get very strict about agriculture and other products that aren't approved for use in-country. They are really looking for truckloads or containers of the stuff. If you don't say "I have something that is not approved for use in Canada" then they'll never know that Mutaflor isn't available in Canada even if they see it; it's just an oversight on your friends behalf. Worst thing they will do is search everything, detain him for a little while and make him throw out Mutaflor. If they suspected that he was trying to resell it for a profit then that is a whole other issue.

Being an American who went to college in Buffalo, I use to go over the border all the time and bring back Cuban cigars. We all know that is not allowed but I still did it; it's no big deal. They'd ask if I had anything to declare and I say no, they'd let me on my way. Now I just have the cigars mailed to me, never been an issue. If they caught me reselling them then I'd be in big time trouble. Do you know who the biggest market in the World is for Cuban Cigars??? It's the US. This stuff goes on every day.