A friend has a bunch of rats in his horse barn. He needs a bait to draw them into a live trap. Any suggestions.

Assuming he's using a live trap because of barn cats and he doesn't want the cats in the trap? If so just use grain or Jelly, Peanut Butter, etc.

Rat traps your looking for disappointment trying to cage them, set the edges they like fig bars. Cover with a box to avoid cats leave a opening

at our farm we have rat issues from time to time in our bunk silos if we are lucky enough a mink will visit for a few days amazing what he does

I did an ADC job like this once. Rats in a horse barn eat horse feed and will refuse all else as trap bait. I did this and it worked: put a 55 gallon barrel with top cut off but placed back on (so rats can go thru the bung) next to some bales of hay/straw. Fill the barrel almost full with horse feed. Let the rats find and eat it for a week or so undistrubed. Cut a round piece of plywood that slips the inside barrel . Cut 2 holes in the plywood big enough rats can fit through. put 6 inches of used motor oil in the bottom of the barrel, fasten the plywood 10 inches above the oil level by drilling holes thru barrel and driving nail thru to the plywood. Put horse feed on plywood (not too much). Rats jump in, cannot jump out and go thru holes into oil and die.