Well, a penis is longer and thicker than a finger, so if it hurts too much to insert a finger then a penis will certainly be worse (as long as your fingernail isn't long or sharp or damaged). It could be because she is too dry (some women naturally produce more lubricant than others), in which case it could really help to use some lubricant (KY jelly or similar), either with a finger or penis.
Has your wife seen a doctor about this?
Well my fingernail is naturally long but its not sharp and damaged. She gets lots of fluid during foreplay so I dont think that would be an issue. Yes we consulted a gynae few months ago and she told us that her vagina is completely OK. We also tried KY Jelly as well.