It certainly is fixable, but it will require quite a bit of work, and of course it depends on how bad the rot is.
The owner says the floor is soft, have you seen it? One of the first signs of floor rot is discoloration of the vinyl flooring. If the discoloration isn't that large, then it might not be too bad. If it's the entire front of the trailer, then that's serious work. Assuming the trailer was parked level, then hopefully the damage is contained to the front part of the trailer, which means there's no appliances to remove and no plumbing fixtures to take apart. You'll just have the bench seat/bed and a cabinet or two to remove. That won't be simple since they are likely screwed in from the outside, but you can do it and replace everything without too much trouble.
The original floor was one piece, so obviously you'd have to cut and put a new piece in there. It won't be as structurally stiff with two pieces, but I don't think that would be a huge problem. People replace floor sections on the older trailers and they still function.
Hopefully the floor section isn't rotted all the way out to the edges. The trailer walls sit on top of the floor, so you'd have to like jack up the framing to get in there. Then you have the fiberglass exterior to deal with- hopefully it wouldn't crack.
If the floor issue is just in the middle and doesn't go to the outside edges, it should be a fairly simple job of just removing the furniture, cutting out the bad floor and removing the wet insulation, then putting a new piece of wood and insulation down, cover with some type of flooring, and then put all the furniture back. If it goes to the outside edges and you have to work from the outside, that's a major problem then. Still doable, but much more work.
I'll caution you, don't just go cutting into it with a saw. Wires and stuff run under there as well as the wood floor joists. Dig in there with a hammer or something and get a small piece out to assess how to proceed.
Unfortunately, it's hard to assess the extent of the damage by just looking at it. You really have to dig in as you go. If the owner will let you though, you could remove the screws that hold the table base to the floor and pull that fixture out. Since it's recessed into the floor, you should be able to pull that out and have a good sized hole through the floor to look at the wood and insulation. If the insulation isn't completely black, that's a good sign. If the wood is still fairly sturdy by there, then that's a good sign too.
Let us know how things work out. We'd be glad to look at any pictures you have of it to assess the damage. That is a very good price for the trailer given the year and model. It won't cost too much to repair for the parts, but the labor will really get you if you had to pay someone do to it. That's why insurance companies mostly just total RVs when they have any water damage.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR