Many jurisdictions approved gas heaters in the attic but the only way of knowing if your sister's did is to call them and ask. Drafts in the attic shouldn't be strong enough to blow the pilot out. It is possible but uncommon for the flue to create stornge draft. The two most common culprits is thermocouple ( inexpensive and easy to replace) and pilot adjustment and/or cleaning ( not reccomended for any besides a professional or above average diyer) .

It'll suck if the plumbing ever leaks.

Water heaters in the attic were fairly common when I lived in Texas, but there were also quite a few horror stories. Leaks are the obvious hazard, but just wait until you have to do a standard replacement. Old water heaters are heavy, rusty and water-logged and you have to somehow get them out of the attic during a replacement. Not a fun job.

The pilot light blowing out sounds like a defect. There should not be enough draft in the attic to affect the pilot light.

Bruce