No, that won't happen. There's nothing on a bike that will take damage in a temperature range that's tolerable to humans. If anything, cooler temperatures slows down decay.
Heat OTOH is routinely used for what's called forced or accelerated ageing tests performed to determine stuffs expected longevity. If you can keep it out of direct sunlight it'll help preserve the paint and everything plastic and rubbery.

Regarding upgrades: Tinkering with a bike has of course an emotional value as well. Replacing an although fully functional part with one that you know is a bit shinier, slicker, lighter (and more expensive) can certainly bring more joy to riding.

Basically there's a chance that you will ride better/harder b/c you know the bike is better.

But you need to be real careful about how much improvement you're expecting, and even more so about how much of it as you attribute to the bike.
As long as all parts are fully functioning, rider ability/effort has a much greater impact on performance.

1) what currency?
2) what was he intending to do?

Not knowing the exact spec of your tandem, I can't promise anything. But it seems likely that you'd be able to fit a rigid, (maybe suspension corrected) 26" MTB fork that can take a disc brake.
Avid BB5 road can be had for say EUR 35, I see forks starting at EUR 60, then you'd need another stem and another headset - say another EUR 50 or so. Oh, you'll need another wheel too.

This is all bolt-on, so it'll bolt straight off again. Unless you misplace parts while they're off the bike, it can be returned to it's original condition (well, maybe with an added tool mark or two) in less than an hour.