Can Loss Prevention Search a Shoplifting Suspect

My question involves criminal law for the state of: North Dakota

My Uncle who is physically and mentally disabled has been stopped, and frisked by the same Security officer at Walmart on two separate occasions. He was accused both times for shoplifting without ever committing the act. While being accused my uncle was forced to spread his hands and his legs, and was frisked by the security guard. This action was taking out in public and the police were never called or on site during the "Interrogation". My questions are:

Was this search legal?
Do the Police have to be present while a frisk is being executed?
Was it legal for the search to be done in public, instead of in a separate room?
Any information on this matter would be greatly appreciated as well as any legal advice.

Thank you for reading.

Was this a uniformed security guard or a plainclothes LP agent?

In order of your asking:
Yes
No
Yes.

That being said, it sounds like it may be a violation of company policy by the LP employee. While I am not familiar with Walmart's exact policies, every retailer I've ever worked for required any searches be done back of house, outside the public eye. Your uncle's legal guardian should talk to the store's LP manager and general manager.

I would be curious to know what your uncle is doing that is causing LP to believe he is shoplifting.

Thanks for the help on this manner. To my knowledge all my uncle does is pick up items to read the information and then puts them back on the rack, util he finds what he is looking for.