If he's in Florida and it's stuck open I doubt it would ever take that long to heat up.. The car will normally always run fine with a faulty thermostat UNLESS it's stuck closed that's more of a problem because of overheating.

In general though normally thermostats are very easy to replace , on some cars you just have to make sure to get all of the old gasket off to have a nice smooth surface for the new one. If not leaks will develop.
I'm pretty sure I located mine just by a glance without even looking it up online , if it is where I think it is it's not that hard to get to. (whenever I get a new car I look for these sort of things it's a common thing to go)

Someone mentioned:
New Water pump after 100k? This is normal routine maintenance????

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2006 X5 4.4i