I needed to convert pounds into kilos, however (18kg) weight loss does appear a little excessive. However, it really depends on how much weight the person is carrying and how the weight loss diet is being implemented. I am on the tail end of a 60-Day Clean Eating Challenge. The motivation for participating in the challenge was not to lose weight or diet but rather as a way to pay homage to my father's passing 12 months ago from prostate cancer.
I thought myself to be relatively healthy prior to the challenge and weighed in at 50kg (I'm 155cm tall) and really didn't need to lose any weight. Following the challenge I'm now 46kg. I eat double what I used to eat (however the food I'm eating or rather choosing not to eat) coupled with absolutely no alcohol or caffeine and most importantly no sugar or processed foods helped with the weight loss. I actually think I look too skinny at the moment and I'm glad the challenge is coming to a close because I don't want to lose any more weight.
I appreciate 4kg isn't excessive like 18kg is however, the point of this answer is that by adjusting eating and drinking habits + exercise + lifestyle choices will enable anyone to lose weight AND keep it off in a healthy way. It's when someone crash diets or chooses a fad diet that rapid weight loss is dangerous.
In summary, 18kg or 40pounds is an excessive amount of weight to lose in 3 months but done properly and under guidance and depending on the original weight of the person is a possibility.