My relative got B1/B2 visitor visa for US for 5 years and it is still valid for 3 more years. She have been to the US twice in the past. She returned to Nepal last time in this February; she again likes or wants to visit US again in next one month.

But I heard that to be eligible for return to the US under visitor visa, one has stay outside the US for 6 months or more. Otherwise, there is chance of deportation from the airport. Is this true? Please suggest what she should do.

That does not sound logical.  If you want to visit the US more than once in 6 months, why would that be a problem?  Especially when you have the track record of not over-staying.

But what do I know.

I dont think staying outside US for 6 month has to do anything with visa.
When you get B1/B2 visa, one can stay in US only for 6 moths unless he/she gets and extension, one can then go outside US and come back as they wish after 6 month stay.

Riten is right but somewhere in travel.state.gov I read that the person entering US should have a clear purpose NOT other than visiting US as tourist (or something like that). Get a reason ready at the port of entry that you are in US this time to attend ___'s marriage, _____or ______engagement, _______'s delivery. To see more places doesn't sound much logical in this case because she visited US twice before. She should be good.

One of my relatives stayed in the US for one year (6 months + extension of 6 months), went back to Nepal and came to US in 2 months, stayed here for one year as before and now went to Nepal last month and planning to come back next month. 

Visa is just an official document to let you inside a country, if your records are clean of overstaying then i think it has nothing to do with how long you stayed in the country before.

Thank you guys a lot; really appreciated. So, this is the summary: one can enter US any number of times under visitor visa, most important thing is that reason for visiting should be strong.