Depending upon the bow 348 is VERY possible even with IBO specs of 30" /70# and a 350gr arrow. The only bow I know of that has a shot at 348fps at these specs is the BowTech Black Knight Dual cam. Both the high brace height version and the low have a spped rating of 350fps at the top end. (The Low brace model is rated at upwards of 350fps with a 29" draw length!)
IF someone shot that speed then there is a good chance that they had this bow........or it could be quite possible that they are shooting any number of the really quick bows on the market at under 5grs per lb of draw weight.

Here' s an example........My BowTech " Pro38" dual cam shot at 70#/30" shot a 350 gr arrow 336fps with a string loop on the string.........with a small tie in nock set I' m sure I' d be at or over 340fps.....and that was with a 6.75" brace height. And that wasn' t even pushing the bow.

ALso my new BowTech Patriot Dually at 30" /70# shot a 375gr arrow at 319fps, and that was with a string loop and SIMS Leeches........and oh yeah.......a 7 7/8" brace height![:-] If I stripped off the leeches, and brought the arrow weight down to 350grs I would most likely bring the speed over 330fps without much trouble.

Your friend could very well be feeding you a line of bull.............BUT there are bows that can do it.