this is so called

adverse possession.

Those conditions your friends needs to satisfy will include:
1) His use of land is notorious so that anybody could find it
2) The use of land is open
3) The use is actual continuous possession
4) It's hostile, the real land owner never permit such use.

The period of possession is different depends on which state you are located. A lot of states will require 10 years.

Note: this is only basic requirement on the law which is subject to many many....limitations. For example, if the land owner become physically unable to claim his right or he is an infant during the 10 years period, the 10 years' period will be towed until land owner have capacity to claim his right.

Don't have muc time to discuss, one more last recommendation: 讓你朋友開始在那塊地上種韭菜吧!

BTW, anyone who took bar exam familiar with the standards I just listed? I still remember it because the Barbri professor gave it to us by the abbreviation "NOAH", remember? :>

Have a sweet dream, all of you!