First, two points need to be clarified. (1) It is not clear from the framing of your query who exactly thinks the supplement should be taken by the friend: the friend thinks this or you think this. (2) This scenario represents an intrusion into the friend's personal concerns, thus is not a matter of exerting the right to a discussion. Now to answer the question in light of these points.
If it is the friend that thinks they need a supplement even though they don't "look" like they "need" a supplement, there is no reason or need for you to intrude and have a "discussion" in the least regard. Supplements have been around a long time now and are closely regulated by the FDA (even new and controversial supplements related to weight and body building are regulated and monitored by the FDA leading to their being pulled from the market on occasion). The position of "friend" gives you no right or reason to have a discussion about taking supplements unless the supplement being considered is one of new controversial ones that could pose health problems to your friend.
In this case, the only way to "have this conversation" would be by respectfully expressing your concern for their health and safety and by suggesting a viable alternative approach to whatever the friend's motivating concern is. Your conversation might provide scientific studies and statistics and FDA information about the dangers and problems with the supplement along with the statement that you don't want it on your conscience that you did not speak up and try to guide your friend toward a safer more reasonable route.
If you are the one who wants your friend to take a supplement, again, approaching the conversation respectfully is paramount as you have no rights in the case (such as the rights a mother, father, or doctor might have) at all. Barring a known threat to health, there are several possible approaches you might take, and each ends with your leaving it with your friend and not pressing for a follow-up as to whether your advice were taken or not, although it is always permissible to say that you are open to continuing the conversation at any time the other might choose to do so. Approaches you might take are:
- your personal enthusiasm and experience with a supplement.
- a simple query of "Have you ever tried (__)?" to which the friend's response will guide further comment: for example, "No, and I have no intention of ever," would put an immediate end to the conversation.
- mention of another friend or a family member who gets good results from a supplement with the comment that you think the friend might benefit from it too, though, like above, the response may call for an immediate end of the conversation.
Two further comments. First, "looking" like someone doesn't need a supplement is not an indicator of actual need (though it may drive felt need) since once a health problem "shows," it may be rather advanced. Second, as friends we always have the right to (a) be concerned about the well-being of our friends and to (b) speak our minds about things that concern them or us (indeed, sometimes we have the moral necessity to speak out). We do not have the right to intrude and demand and coerce and disregard the other's point of view, desires and perspective. Thus all conversations must begin and end in attentiveness, respect and courtesy.