Hi, Logan. I would write a note that starts by congratulating them for saving a life! I would maybe pick up an inexpensive basic dog care book, and some Yorkie size toys as gift for them, too.

Then I would explain that a Yorkie is an intelligent dog, although small, and you are afraid he/she is getting bored being outside all day. I'd mention toys and activities, ask how his training is coming along (maybe they're just keeping him outside for now ...), and how he is adjusting. I don't feel you need to mention barking specifically, unless you feel a need to explain WHY you know he's bored and lonely. Stress "companion animals" and "the pack" and keep congratulating them for rescuing the pup.
Even offer any ideas for good obedience classes that you know of, and say that while he may be housetrained, etc., obedience training helps form a good bond between dog and owner in a way that not much else does. A rescue has special needs, etc., and we are SO glad that saved this dog!

Be relentlessly positive between suggestions. Tell them about the Pet Talk community, and that we congratulate them as well. It's early, maybe we can teach them despite former bad cirumstances!

Just my two cents ...