I see clean, nice Tahoes retailing for anywhere from $9,500 to $15,000, depending on equipment and miles on them. The 5.3 engine is said to be able to go 200K without any major problems, and many go way beyond that. It all depends on how it was cared for. The passenger window could be simple like a switch or more expensive like a motor. A handy person could replace the motor, but I would let a mechanic do it, because I have tried this in the past, and some are not so hard, while some are. For anywhere around an 02 - 06, those miles could be ok, but anything over 100K still scares some people. See if the seller will let you run it by a mechanic for an appraisal of the engine and drive train. As most on this site will tell you, a Tahoe with a 5.3 that has had good prior treatment will be a reliable vehicle.
A private party seller would most likely ask what your saying on the Tahoe, but if I were selling for that, I would have already fixed the window. Even if I had to go and ask something like $10,200 after doing that.
For a reference, I have an 03 Tahoe LS with 2WD. It has 84,455 miles on it. It is totally clean and everything is in pristine condition. I bought it new in 03. While I would like a 08 or 10 Tahoe and new body style, I am so satisfied with the 03 I have that trading is not being considered right now. If all you need is the window repair, and the rest of the Tahoe is upscale, I would not be afraid of buying it. You might try offering him say----$8,995 for it based on the window and 137K miles. You never know he might take it.