There is no standard for the Cantonese pronounciation. Cantonese involved several dialects. The expression of "how are you", it should pronounce "nai hao", anything that is close to this pronounciation is understandable. People who live outside the Canton and/or Hong Kong may speak slightly different. " nei how" and "lei how" sounds like the mandarin speaking people trying to speak Cantonese.

The right pronunciation is nei hou, but lei hou is ok too. There is a trend that Cantonese speakers don't distinguish n from l. There are some others, we call this phenomenon 懶音, you can read about this here (in simplified Chinese): http://www.cantonese.asia/viewnews-246.html

In reference to http://www.cantonese.asia/viewnews-246.html, 粵語協會: 懶音的粵語，已經偏離了正宗的粵語, meaning " 懶音" is seperated from the standard Cantonese So, these people are speaking their native cantonese. For example, 客家人, 台山人, 潮洲人等, they are cantonese too, but their mother tongues are different from those people in Canton city and Hong Kong. They have to learn to speak standard cantonese. Chinese lean to speak English, the difference in pronounciation is called accent, thus cantonese learn to speak standard cantonese, it is also accent.

這是傳統漢語聲韻學中所謂的 "泥母和娘母不分" 即 n l 兩個聲母實際應用時 有並存現象 客家話也有相同情況 如 作 擁抱 摟抱 解的 攬 或讀作 lam 或讀作 nam 皆可

you should use "lei hou".