Just the other day I was contacted by one of my oldest friends, and she gave me the sad news that she was going to be moving into town. Then she asked if I would take her mare Merrida, who is deaf. I've never owned a horse with a hearing defect and I was wondering if there was anything I should be aware of for horses like this.

I have never dealt with a deaf horse - but I can imagine that the mare is used to it - and either is calm enought to not spook at every thing or she might be very spooky at anything that appears suddenly in her visible distance?

I agree, talk to the owner. Go meet the mare (if you haven't already) and gain her trust become her friend. I would also try to use this GS she's familiar with, (I know she's not blind but familiar things are calming) try and keep things the same. Her routine, what buckets she uses, that sort of thing. Also if she is kept with other horses you need to find out if she is okay alone or if she gets along with your horses (If you have any)