waiver of liability in case person climbing on roof is injured

Might not be a bad idea.  Generally, a waiver of liability (aka release, indemnification agreement, or covenant not to sue) is enforceable in Massachusetts so long as it uses clear and concise language, specifically identifies the parties that are being released, is written in a font that is large enough to read, and does not violate some public policy.  See Lee v. Allied Sports Assocs., Inc., 349 Mass. 544 , 550 (1965), citing MacFarlane's Case, 330 Mass. 573 , 576 (1953). Clarke v. Ames, 267 Mass. 44 , 47 (1929). A party may, by agreement, allocate risk and exempt itself from liability that it might subsequently incur as a result of its own negligence. See, e.g., Lee v. Allied Sports Assocs., Inc., supra at 550; Barrett v. Conragan, 302 Mass. 33  (1938); Ortolano v. U-Dryvit Auto Rental Co., 296 Mass. 439  (1937). See also J.W. Smith & H.B. Zobel, Rules Practice § 8.18 (1974). "There can be no doubt . . . that under the law of Massachusetts . . . in the absence of fraud a person may make a valid contract exempting himself from any liability to another which he may in the future incur as a result of his negligence or that of his agents or employees acting on his behalf." Schell v. Ford, 270 F.2d 384, 386 (1st Cir. 1959). Whether such contracts be called releases, covenants not to sue, or indemnification agreements, they represent "a practice our courts have long found acceptable." Minassian v. Ogden Suffolk Downs, Inc., supra at 493. See Shea v. Bay State Gas Co., 383 Mass. 218 , 223-224 (1981).
 
I'm assuming this is a case where the dangers are obvious (falling off the roof) and there are no hidden dangers that you are aware of, such as week spots in the roof or other problems that could create an increased risk to your neighbor.  Waivers of liability are available on line.  For some ideas about the form you use you can check out this waiver that is used by Habitat for Humanity. http://www.hfhplymouth.org/LiabilityWaiver2008.pdf  Good luck (to you and your neighbor).