Sorry you're getting to enjoy this. As I understand it, the prior person broke the bolt, then slipped in a helicoil without extracting the old bolt... they would have had to go up a size or two and cut threads into the water pump for that to hold at all... this sound like a shade tree JB weld "repair".
The only way to make it right, IMO, is to extract the bolt. You will probably have to strip everything back off the front of the engine and you need to be very careful not to get metal debris into the pan... because some amount of drilling is likely.
There are a ton of different bolt extractors available, but I'm going to say don't buy any of them... the cheaper ones, in a size you can use are brittle and break very easily...then you get to drill out the extractor. If it broke off clean, you can very carefully drill the center of the bolt... keep in mind that the last resort is to simply drill out the old bolt and re-tap the threads. If you start the hole with a smaller drill size, you might try a left handed drill bit a size or two up and hope that it catches the bolt and winds it out... do not break the bit... and do use plenty of oil to try to free the threads... I prefer ATF to most penetrating oils.
The most important thing is to take your time... do your best to center the bit and keep it straight. Now assuming you manage to screw that up as badly as I have in the past... once you have gotten through the bolt, the oil gets where it's needed and the bolt usually comes free... you could cut a slot with a hacksaw blade and use a screwdriver to help wind it out... or anything that works... the only rule is the try not to damage the threads in the block. If they get slightly nicked, the tap will clean them enough that they should seal with a little RTV gasket maker. Good chance the threads go into a coolant passage, so I like to seal the threads anyhow (tracing coolant or oil leaks to a bolt you just worked on sucks). Really stubborn bolts will come out once you've remove most of their metal... I have drilled them enough to use a dental pick to get them to wind out or weak enough to collapse enough the threads don't fully engage.
Get creative, be careful and be tenacious... you can do this. Once finished, you'll get to enjoy finding a correct replacement bolt... hopefully not as bad as getting the broken one out.
