I purchased a gold plated bracelet stamped 925 plated over silver. Aperson I showed it to say's it is chaep and not worth what I paid, and will turn the wearers skin green. Is there any truth to their statement?

Yes, 925 gold is 92.5% gold 7.5% copper usually. And some people have very sensitive skin and copper will cause a reaction turning your skin green

Minimum 14K Gold plated Sterling silver is also called as Vermeil. Vermeil jewelry do tarnish.

In Vermeil when the sterling silver base tarnishes it tends to become visible through the gold overlay. To avoid this tarnishing, keep your vermeil jewelry in an airtight bag when you're not wearing it, and use anti-tarnish strips during storage.

If vermeil is cleaned or polished with an abrasive material or solution then the gold plating may get removed.