if you're talking about one of these : http://shop.easternmarine.com/index....categoryID=192
then you won't need much in the way of tools. You can use open end wrench set, or deep socket set (I'd have both handy, just for good measure).
It's essentially a quick fix job ... you put the jack w/plate against the tongue of the trailer -- the other plate on the opposite side -- and run 4 bolts thru the holes in the plates -- cinch down the nuts until the plates start to bend. That's what I've done to three different boat trailers, and they held up and worked just fine.
You really do want to get the ones that will disengage and swivel up alongside the trailer tongue, when trailer is being towed. You will also want to put the jack (third wheel) on the same side of the trailer tongue, as the original one was. Hopefully you won't have to grind down any weld lines that may have been left behind ... but you will want to have a smooth tongue surface, for the plates to snug against. And you will want the "new" jack to be on the same section of the trailer tongue, as the old one was. You will also need to know the dimensions of the trailer tongue, as there are different frame sizes of trailer tongues ... and corresponding trailer jack plates to fit them. Don't worry about the "weight capacity" ratings .... the lightest/cheapest ones are rated for over 500lbs.
Have you contacted the owner, and told him of the mishap ?? He may want a specific brand or style of trailer jack, for a replacement. Fulton or Atwood are two of the more popular, and widely available, brands ... but there are others. Prices range from around $20 to $50 for a bolt-on trailer jack that will swivel up alongside the trailer tongue. All the ones I used were around the $40 mark ... whether from a boat dealer, or Wally World.
I think I would take a good look at the trailer coupler ... the trailer ball should fit snug, but not to the point where it takes jumping on the bumper to free it. There may be a dent in the coupler, that could be causing the problem. IF so, that may take a little more "experienced hand" to correct.
.... luck2ya ... cp