Depends on how much you want to spray. That IS a lot of miles, but with a mild shot and a good tune, I don't see why you couldn't. My car has a hair over 50k miles and it does just fine with a 100 shot.
If you mean risk as in "Will I die when the motor blows up?" Then no, there is little risk to you. It will just nuke your motor and before long you will either have a new car or a lower mileage replacement motor that you hopefully won't be strapping nitrous to again. The odds of it either slinging bits of your rotating assembly into the cabin and hitting you like buckshot or it going up in such an amazing explosion/fireball that you get cooked are slim to none. It might start a small fire and burn the car down slowly though, oil + hot headers = new car if it throws a rod out the block.
If you mean risk as in "Will installing nitrous on my well traveled 3v significantly shorten it's lifespan?" then the answer is yes, there is high risk involved, because that will surely add more stress to a higher mileage motor, and good tune or not, I wouldn't go there personally. IMO, go for some bolt-ons if you don't have any yet, like a good exhaust with long tubes, a cold air intake, maybe some underdrive pulleys. Hell, for the price of a nitrous kit and install you might be able to afford cams. The upside to all of that is the power is always there, rather than the short hit of nitrous, the power can be used around town (sure aren't going to spray going stoplight to stoplight or merging with heavy traffic), and those parts are not going to add any more stress to your bottom end. Heck, if you don't have a good aftermarket tune yet, go get one of those. You might be surprised how much 25hp feels like when your car gains it in 5 minutes. Gears are another option. 4.10s over stock gears will make it feel like you installed afterburners. Bottom line, I'd stay away from the bottle. About the only way I could see it is if you have owned this car from mile #1, maintained it to atomic clock style precision, and never really abused the motor. If those are 100+ thousand miles of pleasure cruising in optimal conditions, with excellent gas and fresh synthetic oil, then you are probably in decent shape. Otherwise, I'd support that motor with mods that will give you some nice gains without risking your bottom end.
Completely disagree, if he wants to have stock like driveability every day and then have some fun racing on the weekend, nitrous is a great option. I just ordered cams and they were more than my nitrous kit. And anything you do to add power will put more stress on components. It's all about being safe with the tune. These 3v's are really tough.
OP, speaking from experience (2nd nitrous Mustang, first was a 98 GT with about 115k miles and a 75 shot from a ZEX kit, my current 06 GT has a 100 shot from an Edelbrock kit), if you have a good tune, you won't "nuke" your motor, it will be just fine. It's all about how it's set up. If you just throw it on and try it out, you will probably end up with a mangled mess, but if a reputable shop sets it up and you aren't racing every day of the week, I wouldn't worry about it at all.