If your friend received a summons, he'd probably be better off showing up (without a lawyer if he has to), than not show up! I'd be worried about letting the neighbor win by default.
I'm not a lawyer but I represented myself in an adverse possession case. It's not hard to research your own state's law on this topic, for free.
Here's just one example of what you can find from simply googling the search terms, "Arkansas," "adverse possession," and "years":
Arkansas: In Arkansas, the duration of such possession is seven (7) years for unimproved and unenclosed land held under color of title, and fifteen (15) years for wild and unimproved land held under color of title. Arkansas Code §16-56-105; 18-11-102-03; 18-60-212. --source
If the land is "wild" as you seem to describe it, your friend's neighbor would have to prove that he'd used your friend's land for 15 years without permission or objection from your friend (or previous owners of your friend's property).
However if the land is merely unimproved, the neighbor could claim it after only 7 years.
In either case, the neighbor would have to prove he'd used the land in a way that is "visible, notorious, distinct, exclusive, hostile, and with intent to hold against the true owner" (source, paragraph 2).
If your friend can prove that he objected to the neighbor's use of the land within the time limit, then he has a good case.
Part of the law about adverse possession in Arkansas has to do with the payment of property taxes. The neighbor hasn’t paid any of the taxes on the property so he can’t claim that as a reason.
Only time will tell. :0(