If he is intending to buy a NIB Ruger Old Army - never been fired kind of thing, then there is nothing like gthe original items. Because of the design they are good for many tens of thousands of shots with any kind of propellant suitable for a BP handgun - Pyrodex, Triple 7, Cleanshot and so on.
IF, however, he is buying one that NEEDS nipples replacing, then Uncles Mikes or Treso nipples are excellent replacements - the Uncle Mikes because they are of the same hex-head design of the originals and also made of stainless steel [so no new nipple wrench is needed - about £6-8], or the Treso because they are made of a strange and forever-lasting alloy of phosphor bronze. The Uncle Mikes cost around $15-16 at my local store in Springfield OR - that's less than a third of the horrendous price you get hiked for here in UK - so if he has relatives in the USA, then I suggest he makes use of them. Treso around $30 or so.
I have no experience of any other make of BP handguns on a personal level, but I'm told that Uncle Mikes or Treso phosphor Bronze nipples are the to-go replacemenents for those of other makes, too.
My choice is Uncle Mike's, since they are actually made in Oregon, a state I support on an annual basis by distributing copious amounts of dollars in many locations during our month or so over there with friends and relations.
tac
PS - never heard of Slix, but if they didn't have a vent-hole in them, they'd be called plugs, not nipples, unless this is something new that I've never heard of. I guess I could look em up and maybe learn something I didn't know.