The Arabian desert (Saudi/Yemen). It doesn't get that cold even at night. I lived in Saudi for 5 years, during that time it once got down to freezing but not below while I was there, and that happened in the middle of the Northern Hemisphere winter. In the Summer, while there is a significant temperature difference between day and night, it doesn't get that cold at night. It got up to 50 or so degrees centigrade at midday (122 Fahrenheit) in midsummer but quite pleasant at night. I'm not sure what the exact temperature was at night but the Saudi definition of cold is 25 degrees centigrade (77 Fahrenheit) and it was still hotter than that. In Britain 25 degrees is a heatwave and everyone rushes to the beach and wears shorts, t-shirts, flip-flops etc. Meanwhile in Saudi when it's 25 degrees all the Saudis start wearing woolly hats and woolly jumpers. Saudi during the night in summer is hotter than 25 degrees. So the Arabian Desert would suit if your alien landed during the Northern Hemisphere summer.
I just googled. Average min and max temperatures for Riyadh in mid summer: about 28 degrees min and 45 degrees max. In mid winter around 10 degrees min and 20 degrees max. ***all those are centigrade*** On one occasion while I was there it got to 0 centigrade (freezing point of water) but AFAIK that was in the open desert, the city's about 5 degrees warmer at night due to open areas losing heat more quickly. The cities were around 5 centigrade at this time and it was considered an extreme cold snap and highly unusual. And it was mid winter. It rarely drops below ten degrees, albeit Saudis consider 10 degrees unbearably cold because they're used to much hotter temperatures. When I told my English classes that in Britain the temperature regularly went into minus numbers in winter, they were horrified and asked how people didn't die. Then I told them how cold it got in Canada.
It's also reasonably flat. It's obviously very sandy and there are dunes, but not hilly or mountainous.
Other deserts in the tropics are not likely to be all that different. Be careful when reading average temperature charts if you're used to temperatures in Fahrenheit as most of the world don't bother with Fahrenheit and use centigrade. 32 Fahrenheit is freezing but 32 centigrade is an extreme heatwave by British standards and most people in most of the world would consider it either warm or hot.
Deathly allergic to snow? Snow's water (if you're allergic to snow you're allergic to water). Maybe the cold will kill your MC however I'd avoid calling it an allergy as this has a specific meaning, related to a specific kind of immune response. Even when talking about humans, allergy should only be used for allergies and not for other similar things, e.g. an intolerance. Usually if an organism dies from being somewhere too cold it would be called hypothermia. It's good that your alien has a different range of temperatures that she would survive at though. It gets tiresome if too much parallel evolution with humans is assumed.
