I want to know what should be perfect diet for my wife she is 5 months pregnant.

All she needs it to up her daily calorie needs by 300. Which can simply be a couple extra pieces of fruit a day with yogurt, etc.
Doctors normally recommend to add 2 to 3 snacks throughout the day to attain the extra calories. Granola bars, fruit, nuts, salads, etc are all great when you want to stay healthy and clean.
But remember to let her give in to her cravings as well. There is nothing worse for a pregnant woman than to not be able to satisfy a craving just because its for something unhealthy.
You might make things worse for yourself that way too, so be warned.

Do like I did. Give her anything that she asks for! Talk to her doctor about her diet and vitamins.