Attempting to boil an inflated balloon is a bad idea. All that'll do is quickly ruin both the balloon and your pan.
However, boiling uninflated balloons is actually a thing. Submerging an uninflated balloon in boiling water for about 5 minutes can result in a balloon that's capable of inflating to a far larger size than normal while retaining its stability. It's not something to do with cheap balloons, though. If you're gonna do it, you'll need a higher quality balloon (like Tuftex or Qualatex). Also, it still ruins your pan.
And while on this subject, baking balloons is also a thing. It is, however, much more of a gamble since, while a baked balloon can potentially be inflated to a far larger size than normal, it is also significantly more unstable. Also, if a baked balloon survives to be inflated past its rated size, it'll most likely wind up popping into countless tiny shreds (kinda like confetti) instead of larger scraps. Because of this, balloon baking is usually only done by people who like to do blow to pops and are after the coveted shred pop. And, much like boiling a balloon, this also still ruins your pan.