I'll bet he wishes he didn't offer to do it for you as well - **** happens, it's not the end of the world and certainly not worth ruining a relationship over. Sorry about the sermon, but sometimes we need it. I'm 71 years young and I've had a few.
OK,if he does tap it out to a sump plug size and fits the corresponding sump plug it will be fine providing he cleans the area thoroughly. As I see it there is no other way and it wont give trouble if done properly.

 

How did you go honkytonkroll - get it sorted?

 

Sure did. Big 5/8"-7/8" expanding plug sealed it up tight. Thanks for all the assistance.

 

Good to hear. I've never heard of those plugs, they may not have them here in Oz, but I'm pleased it has done the job.
I hope you and your BIL are still on good terms.